Optics and bearings are vastly different in terms of tolerance to buildup.
Lenses and Mirrors are vastly different in terms of cleaning requirements, and positioning requirements.
There is no way to fully enclose the optical path, unless you have some kind of telescoping connection between mirrors, because the gantry has to move along the X and Y axis. So you would need something which can remain perfectly rigid (absolutely zero drooping, or optical path gets blocked) across 20 inches, and will collapse down to a few mm. Rigid requirement means thick material, collapsed length means thin segments. Combine those together and you will have an insane radius for your outer segment. This will also be heavy, which will mean you need stronger motors, and likely move far slower. And it is more moving parts that can fail. And if you ALSO want it to be air-tight, that adds even more bulk (higher weight, more radius).
If you want to have air streams at every mirror, then you need to have hoses running out to ever mirror, and reel systems to keep those hoses from bunching up under the laser and on your cut target. These are also going to be blowing away from the mirrors, in two directions. And since they are blowing within the enclosure, some blowback will happen, so you have to ensure that it is 100% dust free, because high pressure streams of dust will scratch mirrors, forcing replacement to be needed. And of course having two hoses of air, and two nozzles, once again means more bulk, so slower movement, and possible stronger motors. And whenever you do still wind up needing to replace mirrors, you will have a much harder time taking apart the case the mirror is in, due to the hoses/nozzles. And you will have to keep the entire assembly completely dirt free throughout the cleaning process to ensure you don’t scratch your new (or newly cleaned at least) mirror the instant you turn the system back on.
Lenses on the openings of cases which enclose the mirrors is the most elegant solution, and it does save a considerably amount of work and technical precision for the user.