Who uses small-arms fire against tanks? I'm sure tanks can withstand arrows and crossbow bolts too, but that won't help in an environment where the enemy has a healthy supply of Javelins.
When your weapon costs >10x more than the weapon needed to eliminate it with near-100% certainty, your weapon is obsolete. Tanks for the memories.
Nobody will use small-arms against a tank because it has no effect. That is the point.
Humans are also vulnerable to bullets yet we still use them on the battlefield. Cost of bullet vs human is probably a bit more than 10x.
There are no perfect weapons, tanks are vulnerable to ATGMs but if used correctly will overrun an ATGM positions. Ita a game of rock papper sciscors but with more than three options :D.
When your weapon costs >10x more than the weapon needed to eliminate it with near-100% certainty, your weapon is obsolete. Tanks for the memories.