It's a proprietary versus open source argument. You can build a mk4 clone for much less than the price of a Prusa-branded one, as everything is open source. The higher price tag is like a donation to open source.
Prusa also makes the open source XL if you want an CoreXY model, or you could go with a Voron which is also open source.
Prusa also makes the open source XL if you want an CoreXY model, or you could go with a Voron which is also open source.