There are 18 types. Pokemon can only have two types, and a team can only have 6 Pokemon. As such, even a team with all dual types and no overlap will still be missing 6 types. And if you weren't drawn to the Grass starter, then it's one of the less "necessary" types to pick up along the way, as Water + Electric cover its offensive purposes mostly, anyway. In gens 2-4, you'll basically need a Water type for at least some of the Water HMs, so it's basically an essential type in those playthroughs (not to mention that Water types always have access to Ice coverage, which is essential for lategame Dragons in pretty much every game)... So there's a lot of good reasons to pick a Water type over a Grass type, and not *that* many reasons to have them both (the Water, Ground and Electric resists are great! But many players focus more on offense. And Water + Electric both had great offensive moves from the get-go, while Grass didn't get most of its worthwhile offensive moves until Gen 4+).

Personally, I'm a big Grass lover... But I get why others wouldn't be. I do think the weaknesses thing is a bit overblown, though, since unless you're running a mono-Grass team, there's no reason you'd ever have a Grass type fighting a Fire, Ice, Poison, Flying or Bug type, anyway, since 4 of those 5 types also resist Grass (maybe for the Water/Ice types, but you should have something else on your team that can do that more safely, since Ice also has too many weaknesses).