Bri Thai pretty much summed up the "why".
As for a stand-up person vs. an untrained person, the stand-up person will indeed try to "avoid" the ground, but likely have little idea of how to do so. This is due to the way a pure striker trains. They train with partners agreeing to keep "at range" to allow them to practise what they do best, striking, but won't know how to deal with someone who doesn't follow that same "game plan".
The untrained person however may realise he can't compete stand-up, so will do the logical thing and tackle or clinch.
A wrestler or grappler trains how to take someone down to the ground though, and therefore how to avoid being taken down also, so while not so good at stand-up, they're better able to keep it that way. That is until they get KO'd at least!
As for groundfighting being less attractive on "the street," that is true, but think how many fights actually occur on "the street"?
Unless you're Superman it's unlikely you'll be able to deal with multiple opponents who mean business anyway, but assuming you're a grappler interested in some strategies, there is indeed material that can be learned for 2 on 1 grappling, but of course they're only last-resort.
Sorry for writing a book!
