Here is an overview of how the procedure looks like for most US carriers: https://thepointsguy.com/guide/buy-second-airplane-seat/
and airlines don’t make it easy.
Not strictly true. On some airlines its straight forward, on others it isn't. The main use cases that the airlines list are here are passenger of size, disabilities, large musical instruments and occasionally "More personal space". It appears the different airlines feel differently about the relevance of these use cases which reflects how much effort they put in to supporting them.
Business wise it has pros and cons: On the pro side: a seat sold is better than a seat not sold, an empty seat doesn't earn miles, and there is no weight or associate or service needs associated with it so it's certainly cheaper to operate than a full seat (at the same fare). The jack pot would be to sell the same empty seat to two adjacent passengers (but in this case you have to manage who "owns" that seat and what you can do with it).
On the down side: they miss out on ancillary revenue and junk fees. Many airlines make the economy experience extra miserable to steer customers towards upgrading (which typically has better margins). For example in the so-called "Euro Business" (https://thepointsguy.com/2015/08/europe-business-class/) the main value is an open middle seat while the cost is often 3+ times that of an economy seat.
People flying together may start to team up and buy a shared open middle seat. The airline may view this as "I could have sold 2 premium seats but only got revenue for 3 eco seats". Fun fact: many couples book aisle and window seats hoping for the middle to stay empty (and just trade it if doesn't). Air New Zealand now specifically disallows this (workaround being to have separate bookings for each passenger)
Why don’t airlines make it easy to buy an extra seat?
Guessing the motivation of airline is always speculative. There is a lot of variation in the actual difficulty so most likely there is no strong reason to do it one way or the other. Most likely reasons are either "not enough of a use case to but effort into" or "our inscrutable revenue optimization system has determined that we'll make 7.3 micropennys/pax more if we do it the current way".