Why Bob and her and not Bob and she?

Both terms can be correct depending on the use. At an early age, we have it drummed into us to say "Bob and I" or "Bob and she". It is not always correct.
Compare these sentences:

We would not say "Her walked towards me" but "She walked towards me" is correct. If Bob is with her, the sentence will read "Bob and she walked towards me". "She" is still the right word to use.

Now look at these:

"I walked towards her" is also correct and therefore if we include Bob, the sentence should read "I walked towards Bob and her". We would never say "I walked towards she".

"She" is used as a subject, the one who is doing something. "Her" is used as an object, when something is done to her or for her. The rules are quite consistent whether it is one person or more than one person.