By charging, they're allowing the people who deem it important enough to pay to sit together, to actually guarantee sitting together. Then slot everyone in who doesn't pay afterwards
But they don't all do this and they don't need to do it, they really don't, other than to make money. If people stopped succumbing to emotional blackmail and refused to pay for seats, base prices would increase.
I got seats for me and DP together for free, with Jet2, simply by checking in quite early (Jet2 open check in 28 days before the outward flight and I checked in maybe 26 days before). I didn't choose where we sat, but we got our preferred option of 2 adjacent seats, one of them by the window.
Someone else might try to check in 2 days before the flight and find that all that is left is 3 single seats scattered across the plane. Now if those 3 seats are all that is left for a couple with a preschooler, we have a problem and they will have to plead with the check in staff or people on the plane to get at least one person to swap - there will be no facility whatsoever in the online check in system to allow them to reserve adjacent seats that simply do not exist.
I wouldn't be jumping up and down to swap but I would consider it especially if we got to the stage of 'this plane will be delayed if this isn't sorted now' I got my free seats by being organised and being aware of the rules. Does that make me more or less deserving than someone else who paid even though they might have been given adjacent seats for free, but they didn't think to check?
I know people should read the rules and T&Cs but do you all really read all the T&Cs when signing up to things? Some of the really basic noddy questions that are asked repeatedly on this site would confirm the assertion that a lot of people don't. Different airlines have different rules for checking in and it can vary from 28 days to as little as 24 hours in advance. If you thought it was 24 hours and everyone else on the flight knew it was 28 days, you've little chance of sitting together whether you pay or not.