A player like that alongside Parker would be an epic waste of floor spacing. The opposing team could pack the paint since at that point the only 3pt shooter on the floor in the starting lineup would be Leonard who is very streaky from three. Then Duncan, Splitter, and Parker have a harder time scoring because one of the wings sucks from three and the other is just decent.
It's a pretty delicate balance. Two bigs, two 3pt shooters, and one ballhandling/slasher (Parker). When you go two slashers, two bigs, one 3pt shooter. . .it just doesn't work nearly as well.
Trust me, Green is an excellent fit. It'd be nice if he could handle the ball better and slash, but as I said. . .if he could he'd probably be getting paid somewhere in the realm of 8-12 mil, not 3 1/2m. He is great for his pricetag. Literally one of 5 players I would deem untouchable if I was the GM of the Spurs. Cost to performance ratio is more important than performance alone man. Especially for a team with a limited budget like the Spurs. You gotta think a little bit bigger.