It's because "본다" sounds not polite and a bit aggressive.
A polite form of it is "봐요".
Here are other variants, all of them are the same in meaning.
1.봐. (not polite, informal)
2.본다.(not polite, informal, sounds aggressive)
3.봐요(polite, informal)
4.봅니다(polite, formal)
\^o^/