I mean they really do cost extra money. Space occupied by lavatories is not available for a paying customer (unless somehow they manage to both get regulatory approval for and convince customers to buy them as chargeable seats maybe) and they incur added weight to the airplane that must be flown around and quite unlike regular seats they require plumbing.

Tbh I'm not at all surprised a 28 (at the highest)-seat economy+ cabin doesn't get dedicated lavs.