Because "pretty much whatever" encapsulates the 99% percent of stuff I'm OK with, and the 1% I'll need to take a firmer stance on.
I allow "pretty much whatever". I think other people, as DMs, should be open to generally saying yes to novel expressions of character concepts. I think if you have a cool idea for a campaign that does require more restrictions, like "Let's do a game where everyone is a warforged druid so we can be Transformers!" (non-hypothetical, this idea was discussed by one of my groups over the weekend), then we make sure everyone has buy-in, but also that nobody should try to be disruptive once everyone is onboard.
We had a question on these boards a couple years ago about whether it was OK to play a gnome in Dark Sun; canonically, Dark Sun does not have gnomes.
As a player, I would never ask to play a gnome, because I assume the DM wants to maintain the canon and atmosphere of Dark Sun.
If I was the DM, I would explain that Dark Sun doesn't canonically have gnomes, but we can absolutely make it work if he wants to be a gnome. Maybe he can be a lost gnome held in stasis, maybe there's a lost tribe of gnomes deep in the Wastes, or maybe just gnomes are actually common on Athas for this campaign. Most of my players would just decide not to be a gnome, and for the few that had a concept in which gnome was essential, would work with me to establish their place in the setting.
Man, this drifted really far from the point I wanted to make of "It's totally cool to use the 2014 druid and the 2024 druid at the same table."