As is seen with e-mail addresses, which are often obfuscated on purpose in the internet, it's a huge problem to just potentially get messages from scammers, due to your address being by default reachable by anyone, since that's what the address is for.

Now think about how easy & normal (not!) it is to change a phone number. Compared to phone numbers, getting scam messages on your mail address is a piece of cake, because usually people keep phone numbers all their life or maybe just change it once, while you still have a chance of changing your mail address. Even if it's a pain in the ass, compared to changing a phone number it's at least realistically possible.