IIRC, enclosures aren't common on consumer models because there's a patent on that design. Not sure if the patent has expired or not but with the trend already in motion, consumers are accustomed to open air printers. Printing the parts for an enclosure is also a good way to break in a new printer.
Yep this prevented any commercial options. It's why the Ikea Lack became so popular as an easy and low cost enclosure to make - I've got one for my Prusa Mini+ and it obviously does nothing to help with venting.
Enclosures aren't common on cheap bed slinger (the ones where the bed moves in the Y axis) because they would need to be much bigger than the print area. CoreXY machines are more likely to be sold with an enclosure.
