Are you watching equipped powerlifters struggling to walk with their knees wrapped to draw this conclusion? Have you seen Owen Hubbard's arch when he sets up to bench 200kg? Seems pretty "mobile" to me.
With the exception of one female with diagnosed hypermobility syndrome, everyone I've coached has increased their "mobility" as a result of getting under the bar, including myself. This increased quickly when I started training, and has stayed pretty much the same over the years since. I haven't noticed any drop off in "mobility" since getting my squat to 600 and my bench to 400, despite entering my late 30s at the same time. My trainees in their 60s are significantly more mobile than their untrained peers. From the conversations I've had with others, these observations are extremely common.