It is a luxury because its not a necessity. Same with having your car washed. Its nice to have, but it doesnt stop the car functioning like, for example, having a service done.
Most people cannot afford a cleaner and, if they are able bodied and neurotypical, are able to clean their homes/do their own washing/ do their own cooking and shopping, and juggle getting kids to school/ homework done/ kids to bed.
Many people do their own home maintenance and DIY, and do their own decorating and gardening. Some do car maintenance too. Some undertake building work.
Most of these things can also be outsourced but its a sliding scale of willingness, ability, time and money. Money probably being the biggest factor.
Most people wouldnt now consider childcare a luxury but in the past when women were mainly SAHM then it would be seen that way, to have a nanny, or to have your child in childcare when it wasnt to do with going to work.
I think the same goes for cleaning. Its probably, in my mind, the worst job on the household running list and IF I had the money I absolutely would love someone to come and keep my place clean. But as with with so many people really struggling financially right now, its not an expense I can afford compared to what I need to spend my money on. That would make it a luxury.