You have lived a very sheltered life my friend.
So there is this thing that "other" people have to do when they need to get somewhere. Its like when you give your friends a ride on your plane, but you dont know them and none of you own the plane. Every person pays a bit to fly from point a to point b. Its known as commercial air travel, and it is as horrible as it sounds.
United Airlines [1]: Basic Economy tickets only include one personal item. They do not include carry-on bags. If you decide to travel with a carry-on bag, you must check it in the airport lobby for a fee starting at $35. If you bring a carry-on bag to the gate that won’t fit under the seat, you need to pay a $65 fee to check it.
As for others ... the gate check fee for Spirit Airlines' carry-on bags is typically around $99 when charged at the gate, Ryanair charges typically around €55-€60 for items deemed too large for carry-on at the gate etc.
[0] https://www.easyjet.com/en/help-centre/policy-terms-and-cond... [1] https://www.united.com/en/us/fly/travel/inflight/basic-econo...
However, another thing that happens is you pay for (or the ticket includes) a carry on bag but because the airline has insufficient carry on space to allow all passengers to bring a carry on, they collect bags at the gate to be checked as cabin bags instead (even if you hadn't paid for a cabin bag).
It's the latter that would be surprising to see airlines charge for, as it's a solution to a problem for the airline that the airline created itself by overselling the carry on space.
You fly jets long enough, something like this happens.