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I can't give an answer which would pass a proper roboticist without them wrinkling their nose, but my guess is that rotary actuators are more simple and space efficient to implement, and can generate the necessary high torques for these systems.

When using a linear actuator, you need to offset it from the joint so that it generates torque (that's part of the reason we have kneecaps, actually; it increases the distance between the centre of rotation of our knee and where our leg muscles are actually pulling from, increasing the output torque). This involves more mechanical complexity in the design compared to rotary actuators, which can just be slapped onto the joint directly. That offset also takes up space, so your system is now less compact.

None of the heavier legged robots I know of use direct drive actuators; ANYmal uses Series Elastic Actuators, MIT Cheetah and (maybe) Boston Dynamics Spot use Quasi-Direct Drive actuators with a low gear ratio gearbox (sub-10:1, I believe), RealHyQ uses hydraulic actuators. Electric actuators simply can't output the amount of torque these systems need to carry themselves around, so they need some form of gearing. The linked actuators you shared are direct driven, so they would need some gearing on their output. I had trouble finding anything about gearing direct drive linear actuators, which leads me to believe it's not really a thing, because if you need high loads, you'll jump to geared linear actuators, which are really just rotary actuators with a belt or rack and pinion!

Aside from the mechanical challenges associated with using a linear actuator setup, it's (to my knowledge) not necessarily better to follow biology when it comes to robots. Wheeled robots can move faster on flat terrain than any animal, so legs aren't ideal when you have access to gas-powered engines or electric actuators. Airplanes with turbines can carry much heavier loads than birds because flapping wings just doesn't scale up to the weight of a jumbo jet with luggage and passengers. Legged robots are actually more energy efficient when they don't try mimicking an animal's gait because they don't have organic muscles arranged in the same way. Any finally, rotary actuators are a better choice than linear actuators (for now) because we don't have hyper-compact, super torque-dense linear actuators that can match our muscles.

This isn't to say that biologically inspired designs can't be better, and there's a lot of research into finding ways of replicating the natural properties of certain materials, such as the strength of silk or hardness of the shells of some crustaceans. But in the end, with the technology we have, rotary actuators provide better torque density, and a more compact and simple design than linear actuators.

[1]: I didn't cite anything here because of time constraints, but it's an amalgamation of half-forgotten knowledge from some research I've done on the topic of legged robots