Because there are different types of bacteria that are vying for domination. The "good" bacteria are fine with antibiotics because they usually only get used when there are too many "harmful" bacteria that threaten the life support system.
Have to sacrifice some of the colony so the host doesn't die. Previously they would have been wiped out by the invading bacteria army anyway
The bacteria developed antibiotics! We just took them from the bacteria.
Wasn't it mostly fungi where humans found antibiotics?
Penicillin did, but I think a lot of the newer ones were derived from bacteria. But doing a quick web search right now I'm mostly getting potential new antibiotics that came from bacteria, rather than proven antibiotics, so now I'm wondering about the actual bacterial contribution...