"People in this replies thread are cheap as hell, wow."
It has zero to do with being cheap. We are discussing whether guests would be offended by being asked for a tip. And let's face it, if it pops up as an option, it makes guests who don't wish to tip feel cheap and offended.
It's nice that you wish to tip your hosts. I don't think that it's a valid reason to have it be part of the platform presented in that way just because you don't always carry cash. If you know you have a check-out coming up ad wish to leave a tip, I don't see any reason you can't prepare for that by making sure you have some cash on you.
Also, you can send money to the host through the Airbnb resolution center by clicking on "Send Money" so there is already a way to do that.
As a host, I don't want my guests to be solicited for tips. @Kris2933