Because it's an illusion Janice. This isn't conspiracy talk here...it's reality. Lets look at this more closely and hash out this flexible concept:
* I buy 1000 shares of company A and my broker credits those shares to my account. There is a digital certificate showing I am the owner blah blah.
* My broker immediately loans out my 1000 shares to another client wanting to initiate a short position. When that short seller puts the shares onto the market, some buyer(s) somewhere buy them.
* In theory, there are now two or more owners for the same 1000 shares.
* I later sell my 1000 shares to some other buyer and my broker closes my position. The digital certificate is moved to whoever the new "owner" is.
* My broker is now "missing" 1000 shares and either needs to buy them or wait for the short seller to return the shares.
There are billions of transactions occurring all the time and the complexity of keeping all this in line in real time is not possible.
The brokers can and do walk a tight rope and often get greedy. This is how the market works and it has only become a public spectacle as a result of the GME and AMC et al debacles. THERE WERE MORE SHARES SHORTED THAN THE ENTIRE FLOAT. FACT. It has been going on for ages and will continue to occur. The only difference now is that brokers are shadily locking their clients out from buying when a squeeze is possible.