Because getting out people from 400ft away can short circuit an entire adventure. If teleporting people just about in reach is a 7th level spell teleporting ones in a pretty long distance should be at least upcast to 8th, possibly 9th level because it doesn't make the spell better it conceptually changes it.
If your limit is "this spell can short circuit an adventure" I have some very very bad news for you about magic in DnD. /s
More seriously though, yeah, you are again describing EXACTLY what metamagic should DO! Again, the entire reason that sorcerers have fewer spells, less access to spells, and worse spells than the wizard is because they are, how was it just put? "
Sorcerers are specialist who focus on a small selection of spells but learn how to make them do more."
That is the drum that has been beat for 10 years, and here we have an example of just that, taking a spell and learning how to do more with it, to the point it is capable of doing something new and exciting.... and therefore we can never allow the sorcerer to do that?!
And you seem to be forgetting the other limits of Teleport. Namely, the number of creatures it can target does not change. So, at BEST this is a sorcerer able to teleport a scattered party away from a threat... this is not only good, it seems like it could be a campaign moment where the sorcerer can actually shine for being uniquely capable of doing that, rather than "well, I blasted harder in combat"
But both fundamentally do the same thing rather than fundamentally changing the use case.
And teleport still... teleports. It just allows you to grab people who might have otherwise been out of your reach. That is still fundamentally doing the same thing, just like hitting a 20 ft area for 3d8 is the conceptually the same as hitting 100 square ft area for 12d8 or whatever it is.
Because it's a warlock example and Eldritch Blast is on the table. And EB isn't in isolation much better than firebolt (yes Force > Fire but it's pretty trivial) but takes damage boosts and damage procs better than any other cantrip.
None of which talks about Poison Spray, but since your argument rests solely on "But Eldritch Blast is the only possible choice" then we had best stop here on that tangent.