I can do that for MY cats, but I can't do that for the fosters that I
have
90% of the time.

Why not?

Because she has no ownership rights to the cats, they belong to the shelter
she fosters for. Up to shelter if they want to microchip. If it's not a
no-kill and they euthanize, it would be a waste of money for them to
microchip an animal that doesn't get adopted (sad but true). Small shelters
may not have money to microchip. I foster for a no-kill, they microchip the
day they get fixed.