Why the Army cannot simply supply Marine tanks

Why cannot the US Army provide tanks when the Marine Corps needs them?

It can contribute, but its armour may already be committed to its own operations, so dependence on it is not a guaranteed solution.

The Army should retain tanks and cooperate with Marines, but its armour has independent missions and may already be committed when an amphibious force suddenly needs protected firepower. Depending entirely on another service therefore creates a gap precisely when both services face simultaneous demands. A roughly 60-ton tank also consumes substantial amphibious lift. The choice involves both interservice availability and the burden of carrying heavy armour to shore.