Why do my tires always deflate between rides?
First of all, do your tyres (tires) lose air at the same rate or are there noticeable differences, one going down faster? If this is the case the last paragraph may explain things.

All tyres lose air over time. However the bigger the tyre the longer it will take to notice the loss. And also the higher the pressure the faster the loss. Your car wheels have the same drop of pressure but because of the bigger volume and the comparatively lower pressure the drop is less noticeable. Still you're always advised to check air pressure each other month.

Pressure drop is mainly due to permeability of the rubber, let's call it like that, of tyre and inner tube which are constants for a specific type of compound. In the late 80s latex inner tubes came to use because they were very light and more resistant to perforation. The disadvantage however was that the pump came into daily use.

The quality of the seal of the valve plays another part. It is quite normal to top up a 25mm racebike tyre @100psi at least once a week. All Schrader (car type) valves have a removable core. For many newer Presta / Sclaverand valves it's the same. To do this you'll need a special, inexpensive tool. For some factory fresh valves the core isn't properly screwed in and may leak. It's always a good idea to check the tightness of the core, it could be a reason for leaks.