Because they have a ton of users. People who don’t have credit card numbers saved on autofill so PayPal is more convenient. People also think it’s safer because the website doesn’t see your card number
I struggle to see how it is convenient. At least on Android, when you pay for something, it doesn't use the installed app, but makes you login in the browser. And they still don't support 2FA or Passkeys properly, making me to receive an SMS almost every time.
I can't answer that because I rarely use paypal myself. In fact they banned one of my two accounts (personal, business) for using gift cards which makes me uninterested in using the other one more than once a year.
That aside, I've seen many normal people use it for the reasons I described: They don't want to type in their card number and they think it's unsafe to type in their card number. It's (literally) all in their head so don't think too hard about it.
The practical alternative is Apple Pay, but you can't use it on Android.
Because they have a ton of users. People who don’t have credit card numbers saved on autofill so PayPal is more convenient. People also think it’s safer because the website doesn’t see your card number