It might be in the expansion, it's hard to tell. But I imagine the reason is to give people a reason to raid for them
With starters I can understand them just giving it to them, especially because they should have had those forms at launch. What I'm saying is for every other Pokémon once you raid for them and catch one, you can then "unlock" the Gigantamax version throughout that series of Pokémon. That way you still have to hunt for them and shit but once you have them you have them.
Like there's a lot of Pokémon right now that I feel silly investing so much into in case they get Gigantamax forms in the future. Which is like the exact type of shit that has prevented me from getting really into Pokémon late-game that I thought SW/SH finally fixed (not being able to raise the Pokémon you really want in previous games due to set values and shit). They fixed so much but just threw in another hurdle.