What would be the point of that? The receiving team gets it at the 25 whether the kick went to the 20 or through the end zone.
Because a high and short kickoff where you aim for the line creates uncertainty, and you have to field it or else it's an onside kick.
A guy running up from the 5 or 10 to the 25 to catch something isn't as easy as it sounds. Lots more chances of muffs and you have to actually catch it for it to be a fair catch. Of course if it's a blocker that has to catch it, well again, things will happen occasionally with that.
If the returner lets it bounce crazy stuff happens. Even if you secure it most times, you won't necessarily get it back to the 25.
The difficulty will be getting the lift on shorter tees. It would have been smart for the ST coaches to have lobbied for higher tees as a concession. Maybe it still can in future years, but doubtful since NFL doesn't want these to be plays that do anything now.
Additionally, it might alter the formation of how many and who/what you put back there.
That then of course leaves you open to an actual on-side kick.
Like I said also, squibs are immune to fair catches, so I'd expect alot of them to be squibs.