I suspect there's another reason: the models themselves:
I know a lot of people do use other sources for their beats, however I still see the vast majority of people buying the real models. As for the khymera a lot of people use the old ones, but the current models are still extremely beautiful models. The big obvious downside is the cost of the beast models. The Clawed fiend is actually also pretty nice and the razorwings are pretty nice as well.
Now the 2 obvious obstacles are the size and cost of the unit. The size reflects the fact that this squad is usually composed of Khymera and razorwing flocks, + beasmasters. Several models, a lot of cash, even if you do use hellions and chaos hounds for khymera you're still forced to find the razorwings somewhere else.
The grenades aren't such a big problem, the Baron isn't a must either, I still haven't used the baron deathstar unit, my beast squad does fine on it's own, anyone saying the opposite is talking crap, this is a retarded deathstar unit, it's just so powerful and it's not fragile at all.
As far as I'm concerned, price is what stopped me. Having to buy so many models to compose one full squad, it's pricy and troublesome. You take warriors, one fairly cheap box and you got a whole squad, a second box and you got your transport.
Painting the squad shouldn't be underestimated either, it takes a lot more effort to paint so many variable models. This takes way more time than painting simple wyches or warriors.
It's true that I haven't seen anybody with more than 1 beast squad. There are 0 doubts that I'll make several squads in the future and usually when I have access to some weird armylist simply because I got the models, I usually try stuff out. It's pretty clear that I'll need a 2,500 pts armylist.
I got 6 FW raven models (for apoc and simply because it's a dark eldar model), back in the previous edition I tried out out 3 of them in a couple of games simply because I could.
Another obvious reason why you don't see 2 or 3 beast units: not everyone has even 1 beast squad :p.
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