What airline is this? I haven't noticed this. As long as I get the tickets early enough, there are always window and aisle seats available.

I think it depends on the airline. Most of the budget European airlines I fly with will allocate you a middle-seat by default if one is available when you choose the free "give me a random seat" option at check-in. I presume this is to give away their paid seat inventory as slowly as possible, and also because by giving the passenger the worst available seat you get more value from paying to choose your own seat. Of course: they have a limited number of middle seats to give out so I've found that a good strategy is to check-in five minutes before the online check-in cutoff time which usually yields window/aisle seats and frequently (~30%) exit-row seats.

I prefer window, and I get window basically every time.

Airline "Southwest" allocates most seats based on when you click "check in". If you check in earlier, your boarding position is closer to first, and your chance of getting a desired seat goes way up. You can pay extra to board earlier ("Early bird") for $15. They don't have classes (there is no "first class" or business or coach) but you can pay for other features like cash-refundable tickets or boarding first.

Airline "Delta" simply lets you book your seat, so if you buy reasonable early, you can book a desired seat.

For other airlines, if the cost to upgrade to a window seat is on the order of $10-15, then I do it.

So I think your experience is limited to a small number of airlines, and your description of the OP as "childish" is poor.

Southwest Airlines early check in is $35 at ticket purchase time or $40 at the gate. When flying with my family that’s an extra $200. I like southwest but this is one thing I really dislike about them.

Perhaps it depends on the flight; mine are always $15.

See https://www.southwest.com/early-bird/

What airline charges extra for non-middle seats? I’ve never seen that.

Any airline that sells Basic Economy tickets (without seat pre-assignment) does by allowing seat choice for Economy tickets and above.

In fact, Frontier charges more for the middle seat. I assume this is because middle-seaters are likely to be a couple/family wanting to sit together and forced to pay the premium. No solo traveler would willingly purchase a middle seat, so you don't need to optimize for pricing the middle seats for them.

Seems this wouldn’t work very well. Pick the aisle and window; middle will always trade IME. If you need three across, book aisle, window, and a nearby aisle and you again have tradable seats.

Or book aisle and aisle for a couple.

Maybe they’re just picking up a few bucks from people who aren’t this clever.

I’ve seen this a lot more over the last few years with US carriers. The bottom tier ticket doesn’t let you choose and you have to pay more to get a choice (aisle or window).

It definitely used to be more of a first-book-first-choice policy but now they’re using it as another upsell.

Air France and KLM and Lufthansa don't charge more for aisle/window.

This is categorically untrue.