Why do people have to build their own coils? Do they have to do some DIY, or can you just buy a vaper off the shelf?
I don't need to know Ohm's Law to use my cellphone, etc. Why for this product class?
But building coils is a hobby in its own right - there's lots of different variables you can tinker with (coil diameter, number of turns, wire gauge, wire material, wick material, wicking style, multiple coils in parallel, ...) and get slightly different results. It's fun to experiment, and there's a sense of achievement when you get a complicated build working just right. It appeals in the same way that overclocking your computer does, for example, or tuning your car/bike engine.
Also, all those cartridges will eventually be discontinued as the manufacturer moves on to new devices - but you can keep using a rebuildable device as long as you can get suitable resistance wire (Kanthal A1, Nichrome, or a certain grade of stainless steel) and wick (organic cotton, cellulose fibre, or silica yarn). And those raw materials cost basically nothing per coil, compared to a few dollars for each of those cartridges.