I guess I am ignorant, what would the obvious reasons be, eye candy? You have Maryse, you don't need any more than that. #problemsolved.
But in seriousness, add more valet's and manager's, it should be done anyway.
It's all to do with the concept of "what a WWE wrestling show is".
A WWE wrestling show will (usually) have a women's match, a tag match, a gimmick match, a 'giant' match, a title match. This is what you get when you buy a ticket to a WWE (house) show, and they're going to deliver it. Similar logic applies to the TV shows.
The women can attract a difference audience than the men do. There are a lot of young girls who see the likes of Bayley and Becky become massive fans because of them. That roll can't be filled by a male wrestler, it's just impossible. And you want that to happen on both of your shows, as well as make sure you don't disappoint fans when they go to a house show that is from the non-women show.
The quality of the division is very much a secondary concern, so they're on both shows.
Eye candy is also a reason, but like you say, you can solve that with characters like Lana or Maryse easily. Back in the day, this would be fine, but it's a very different WWE now. They want to attract new fans with their female wrestlers, not just give fan service to teenagers who don't have an internet connection like was the case in the 90s.
You also have to remember that WWE are a very quoter based company. They actively make sure they have have representation of everything. They always X many black people, X many Mexicans, X many Women and so on. Like that's the entire reason you have Saxton and Otunga on commentary, they're terrible but fill their quoters, it's not like the WWE think their good or anything.