okay so this is in fact the crux of the problem. i.e. there isn't in fact a problem, but just the perception of one.
it is the choice and extreme availability of the choices which makes it seem like there's only 1 good book in every 1000. in fact, people have already published ABSOLUTE rubbish and indeed since the very earliest days of publishing there have been poor books. badly written, badly spelled, badly conceived, badly plotted, nonsense. they are Immediately forgotten, never got mentioned outside of their little town, etc. the badly written books from the last 10 years, on the other hand, are on a small printrun of 20,000 copies (not 500), and additionally, are still around - e.g. in your local bookstore or charity shop or social media feed - where you can fret about them.
also (1) - you're worried about whether there was a truly life-changingly good book published in 2025? maybe there was a book that would appeal to YOU specifically as Excellent, and it was written in Korean or Telugu or Bulgarian and it didn't get noticed enough to get translated. maybe this Korean masterpiece did get translated, but only into Japanese and Indonesian. anyway, you will never know. but equally, this may have happened in 1834 and you STILL wouldn't know, unless it got re-discovered - but this in itself takes time, and clearly only a very exceptional book will be still praised 100 years later, at home and halfway around the world.
also (2) acting like the 18th-20th centuries were great for novel-writing and that the 21st century isn't, ignores the situation in much of the world. many "smaller" nations and/or colonised nations, had not started writing Novels in earnest before the second half of the 20th century. novels aren't the only means of cultural expression in the literary field, btw; e.g the first Kyrgyz novel is Uzak Jol (The Long Road), by Mukay Elebayev, which was published in 1936. the next major novel from Kyrgyzstan is Jamila Chinghiz Aitmatov, which was published in Russian in 1958. but they had the Epic of Manas, of course; in contrast, today, about 900 novels a year are published in Kyrgyz, about 60% of them in their own language and about 40% of them in Russian. clearly this is a very robust publishing industry, where it wasn't before. but if you are an English reader you can overlook this situation, and many similar ones across the world's 200 countries very easily.
also (3) we have a tendency to collapse the time periods of the more distant past and experience that past as compressed. oh, there are sooo many great 19th century books? okay, but name a specifically excellent book from 1870. hmm ... maybe it wasn't a 'good year' - or maybe world-class books are rare enough that there's only a dozen or so per century, and therefore it's hardly alarming if you, personally, haven't (YET!) read something extraordinarily good published 2016-2026 (for example). but we valorise the 19th century as the epoch of great literature on the basis of perhaps 40 truly incredible (European) books.
also (4) the whole function of 'Classics' is that the filter of 'educated' public taste has latched onto, and remembered, and analysed, and kept in print, those books which were thought to be exceptionally worthwhile. this cultural filtration needs time. especially if you are waiting for literature written in X country to be appraised domestically, and then gain international notice, and then get translated into your native language. of course the average book you are told you Must Read from 1900-1910 will be better than the average book you would pick up if you chose at random from the list of books published (in English) this year.
also (5) lock in and read a bit more, and you will find (if you read exclusively from the 21st century, rather than the 19th + first half of the 20th) that a. you will find great novels you wouldn't've had otherwise but more interestingly, b., you will discover that, when you read 'classics' more or less exclusively, you train yourself to see many superficial elements of these books, such as the language of the time, as hallmarks of quality, whereas (much of the time), the language may be just older - if you attune yourself to a the contemporary time period again, you may find poetic quality in a different style, and grow to appreciate it as well. MONOTONOUS DIET = NOT IDEAL. Tolstoy is beautiful. it is not the only way to write. (correspondingly, if you read enough 19th century prose you'll stop idolising it and realise that some very good books from the 1860s are also written in fairly unremarkable style, which may not be apparent if you persist in the unconscious assumption that older-sounding = better).
also (6) there's nothing ruining books from the 2020s partly because what you're railing against is most of all, mass literary - and the mass visibility of literacy. people will want to read salacious, trivial, and same-y things. it is okay. you're just more aware of it now because they're saying so out loud, on tiktok / tumblr / etc. we're also writing unprecedented amounts of surreal, formally innovative, very politically enlightened, very culturally rigourous books. go and find them.
exercise for the reader: make a list of your top ten favourite books and notice how far apart they are chronologically. are you expecting every book you read to measure up in some way with your top ten? is it particularly likely, statistically, esp. if you avoid newer titles, that the hypothetical 11th book on your list would happen to be from the last three years? out of all of 3 centuries of modern publishing history, and the preceding centuries we have ransacked for things to put in print?