two reasons: in daz the models are heavily geared towards looking like 23yo fashion models, and making them looking decent 40-50yo is incredibly difficult. you can make them look 80yo or 10yo but a hot 40yo is so hard. so mostly you're gona see either 23yo daz moms or hideous older ones. older + still hotlook is just very difficult to achieve in daz unless you sculpt your own models or something.
the other reason is the dev doesn't actually like milfs but still wants to get on the bandwagon, so he'll make them look about the same as the young women which he actually likes but claims they're middle-aged. it's always very disappointing. in written stories you even see them writing character breaking descriptions like: "Mom still looked so young she could easily pass for a college girl" which is just catastrophic for a milf story.
I'm not a big fan of making characters look middle-aged by using wrinkles and saggy tits, but those have their audience too. I like the more idealistic 40-45yo milf with big full tits like christina hendricks from mad men, sofia vergara or sophia loren in her 40s.
I mean I don't think the milf fantasy was ever: "wow dude, your mom looks so dry and wrinkled I want to cum between her deflated saggy tits!" It was always: "dude, look at how your mom's huge round melons jiggle when she walks!" right? older, thicc and gorgeous.