I suspect this has to do with aracnophobia mods being relatively easy to understand and accomplish, along with the need for it being well-known.
The average person has enough understanding of the concept of intense, specific fears (of which arachnophobia is a common go-to example) to realize that someone with arachnophobia will probably have their enjoyment of the game ruined by a spider monster jumping out.
The solution of just getting rid of the spiders is fairly obvious, and swapping one model out for another is, to my limited understanding, one of the simpler ways to modify a game.
Seizures and what causes them are less common knowledge, due to not being referenced in something like every third piece of children's media.
Additionally, even if someone has a basic understanding that flashing lights can cause seizures, that doesn't translate directly into automatically being able to tell what needs to be adjusted. It's entirely possible that a person may not think to count something as flashing lights that produces the same effect when displayed on a lighted screen.
I can't speak for the difficulty of adjusting visual effects, because I know very little about programming, but making a mod to remove seizure triggers would at least require research and a great deal of conscientious double-checking to avoid accidentally creating something leads people to believe it is safe while still containing the danger.
The motion sickness thing is complicated because of how motion sickness works and how much it varies from person to person.
Motion sickness is actually a feature, not a bug. It's part of the human nervous system's self-preservation instincts and functions as a defense against food spoilage and other ingested toxins.
When your brain receives sensory input that doesn't make sense (such as one sense indicating movement while another reports stillness, or visuals warping strangely), your survival instincts assume you have consumed a neurotoxin, and respond with nausea in hopes of minimizing damage done by getting you to purge any not yet absorbed.
What specific combinations of sensory input cause motion sickness and how severely varies from person to person and even within an individual at different points in their life. This is impacted both by genetics and by how a person's experiences have trained their nervous system.
Time spent rocking in rocking chairs, for example, has been shown to help people become less intensely prone to motion sickness.
Each individual's experience is going to be different. Some people prone to motion sickness may only have difficulties when the camera or display does something weird and unexpected and could be helped by something as simple as removing a filter used to indicate certain effects, whereas others just can't play first person pov games at all, and accommodating them would require re-making the entire game with a different type of game play.
There genuinely should be more done about accessibility in video games, but I don't think people are actively choosing not to accommodate other conditions in order to focus on aracnophobia.
I think someone, somewhere, made a mod for a game because they or someone they knew needed it, and other modders saw that and realized making something similar for other games was a relatively low effort thing they could do that would help others.
The existence of aracnophobia mods does not inhibit the creation of other accessibility mods. There is no reason people should have to choose only one type of accessibility to mod for.
I can understand the frustration of important issues going unaddressed, especially when other issues are being handled, but shaming the compassion and helpful spirit already on display is not the way to encourage additional helpful behavior.
Generally speaking, no one with special needs is helped by being pitted against each other, or by the idea that people should be selective and exclusive about who they treat with compassion.