why views items per page set to specific values
To be brutally simple: Because someone thought it to be good UX, maintainers agreed, and no one convinced them other way since then. That's all to it.



There are many reasons for drop-down instead of text field. For example simplicity. User cannot enter bad values, so there is no need for bad value handling. Testing - you can programatically set other amounts, all right, but there you have ones they tested their module for, and take some responsibility that they work properly. Of course, maybe someone simply liked it that way. No way to know for sure what was the most important part in this decision.