Because it's very easy to turn a 130 dollar barrel into a useless hunk of metal if you don't know what you're doing.

If you're a hobbyist or a tech, you'll probably figure out that milling parts just requires the right tools and a little youtubing.

Also most stock barrels fucking suck ass. My arp9 barrel was so bad... even though it was somehow shooting consistently, it was brutally bad. One of the worst barrels I've seen barring my LCT barrel (which had scratches near the fluting)

But yeah. Barrels have to be precise and free of any marring or scratches or you'll get side spin and shit. If you don't have a pipe cutter, deburring tool, and tbh a crowning tool, you really shouldn't do it. That being said you can buy ALL of that for less than the cost of a decent barrel.