Why does the PSI in my bicycle tires drop from 75 to 50 each day?

A slow leak from a puncture is most likely. Not as likely would be a leaky Schrader valve (but that would be easy to check with soap and water).
 
there could be a lot of reasons! you might need a new tire tube or you could have a puncture in your actual tire. also, you could have leaks around your valves, sometimes sealant will help that problem. you may have leaks around your rims, etc. this link might help you:
 
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