You can deposit in Bitcoin but can't withdraw it
You can neither deposit nor withdraw bitcoin from PayPal.
but you can spend it buying other stuff
You cannot spend bitcoin using PayPal. If you want to buy something with your bitcoin, then you must first sell that bitcoin back to PayPal and then buy the item using fiat. The only thing you can do with the "bitcoin" you have bought on PayPal is sell it back to PayPal.
While PayPal is a global company they must have some reasons for not allowing these actions!
Bitcoin is their competition. If bitcoin payments become widespread online, then the need for PayPal greatly diminishes, and so too does their profit. Make no mistake - they are not doing this because of some desire to aid bitcoin adoption or to make bitcoin more accessible to more people. They are doing it for one reason only, and that is to make profit from the high fees that they will charge.