There are two answers.
1) Your muscles don't care what the resistance is. Chains, kettlebells, rubber bands... they don't care.
2) Air machines are a fad. Loads of kit has come and gone but weights have stayed.
gyms typically will charge members an extra fee and use that money to bring in some equipment or swap some out
it is a good idea for gyms to do this so members who are year round don't get bored
as for your question, there is always machines in the gym that nobody uses that just sit there.. it is a trick called taking up space to make it look like the gym is utilizing every part of the gym