Also worth considering that the Tau likely have many more battlesuits than the Imperium has space marines. Your typical Hunter Cadre (think of the Tau equivalent of an Imperial Guard regiment) has around 10% of its force composed of battlesuits. Aside from the obvious benefits of integrating your elite heavy soldiers into the existing regiments instead of having them exist as a separate organization of autonomous warrior monks, there's also the fact that, to put it simply, they would have to produce a ton of battlesuits to make such a thing feasible. Which, logically, they presumably are. A battlesuit after all is just a piece of equipment that you can stuff a veteran soldier into. A Space Marine has to basically be trained from a very young age in a process that kills like 99% of the recruits that go into it for MAXIMUM GRIMDARK.