It's about the identity and usage. Titan least played class in the game. Void titan would lose the 1 thing it has on the other 2 while invis and devour exist and is available on all classes it's not the thing that makes them unique or rather you don't play void warlock for devour you play voidlock for the massive control potential and you play hunter for the 100% uptime on invisibility you can never come close to 100% uptime on invisibility on titan and warlock. Is it weird that void OS is titan only? Maybe. But it's the only thing that makes it unique and also bastion got nerfed by 60% which is insane

Because it would be cancer. Invisibility, overshields and devour on all three classes. It would be broken and Bungie would nerf it into the ground. There is a reason Invisibility is tied to finishers and Devour to Orbs while Overshields are designed strictly for the Titan. Players are already complaining about how oppressive and obnoxious Invisibility and Overshield spam is already in PVP and this would be a nightmare to address and deal with overall between players and Bungie collectively. Players would spam all three concept excessively therefore killing the power Guardians are suppose to have in PVE. Bungie even stated that some stuff cannot be tuned separately and this is probably one of those features. This is why Heart of the Pack should have been brought back because it gave you a version of Overshield without the shield plus Haste and Recovery.

Solar warlock middle tree can cast overshields

That's because they want Titan's to be the tank class remove that and instantly Titan's lose void identity

If you play all three classes you should understand then part of your post is wrong, warlocks on all subclasses can get overshields(especially middle tree solar) along with stasis, not too sure about Hunters though.