Couldn't you just pay a breeder for a larvitar or frigibax if you want one of them so badly?
Technically yes, but people breeding any of The Big Ones are pretty rare since most of them breed pretty infrequently or some behavior that makes trying to breed and raise a bunch of them a nightmare, so they're not a super popular choice for breeding unless you're passionate, willing to wait forever for eggs, and have a lot of money to sink into the whole operation. Most are also incredibly popular with high level battling so the limited spots breeders have tend to be filled up quickly. I think I mentioned knowing a larvitar breeder in Cyllage whose waitlist is already full for several potential future clutches? And also they're just so expensive to buy and take care of but that's not something I'm too worried about. Like yeah I could catch a larvitar in the wild, but I don't want to deal with everything that comes with that. One from a breeder is already familiar with humans and pokéballs and all that.
The thing with tyranitar is they only breed once every 2-3 years. In the wild they travel long distances to a mating site in the mountains (not always their home mountain) but in Kalos they actually prefer coastal cliffs. Once they get to the mating site they either find a mate if they've only reached maturity that year, or they perform a special call unique to each tyranitar to find their mate from previous years. Yes, that's right, they are monogamous, and mate for life, even though they spend time outside the mating season completely solitary, for some stupid fucking reason. This means that not only do breeders have to wait years between clutches, if something happens to one of their tyranitar, they have to replace the whole pair or get creative.
Baxcalibur on the other hand usually breed once a year. They form harems with one male and usually at least 3 females, up to a maximum of 7. 2 females can work but they prefer at least 3. Females will eat their eggs and leave a male, if they didn't already reject him before breeding, if they think there's not enough female baxcalibur in the harem because they take turns having 1 or more females depending on the harem size watching their eggs while the remaining hunt and the male defends his territory from other males attempting to harm or steal his mates and offspring and any other threats. This means that if you want to breed baxcalibur you need at least 3-4 of them, and for reassurance in case someone rejects or doesn't get along with someone, potentially more. THAT'S A LOT OF BAXCALIBUR. Females also only lay 1 egg a year so it's not even like you're going to end up with a million babies to sell every year.
So basically breeding them is a nightmare which means getting them from a breeder is also a nightmare.