I think it's the competitive "race" element of it.
I am not a mummy martyr by any means, I haven't spent 10 months welded to my baby's hip or anything.. I've been out a few times, had weekly two hour driving lessons etc. Recently I have started to go out to exercise classes and I enjoy the time away, soon I will return to work etc. It is nice to feel the dust settling a little bit, though I am under no illusions that my life now resembles my life before and I do agree with the OP that I have no real desire for it to.
In terms of the phrase, I have found there can be unrealistic (and sometimes unreasonable) expectations of life post-baby e.g. a friend who was irritated by having to wait until the six week check to resume running (she wasn't previously an athlete btw), friends who take dcs to fancy restaurants at inappropriate times just to prove they can and - to a certain extent, and I know I will be flamed for this - people taking e.g. taking mobile babies to muddy festivals. I just can't see why you would want to do all the stuff you did when you were free and young and spontaneous complete with nappies and pots of food and an unwieldy pushchair. It seems like a lot of stress to be able to tell your pfb that they saw Iggy Pop at Glasto etc. I saw an article about this online where one of the comments beneath said something like: "your child is writing their story already, how fantastic!". Their story, how so?
Obviously, it is different when the discussion relates to older dc who are going to school/teens etc. However, if you are one of the majority who will have two children, in the very short time that you have babies/kids under 3, I don't think that, in the main, it is reasonable to want an exact replica of your pre-baby lifestyle if that involved heavy boozing/partying etc. Someone I know has a 3 and a 1 year old who are in full-time creche 7-7 five days a week and every weekend, they deposit the kids with their grandparents and go out boozing - out Friday night to cinema etc, all day drinking session on the Saturday, home at 4-5 am, back with the kids for a very hungover Sunday. I don't care if writing that sounds judgey, because I do judge that kind of extreme behaviour.
I suppose it depends on circumstance and context, as with everything. It's not unreasonable to expect that you should get a life back where sometimes you get to watch the TV or go to the gym or enjoy a meal when the kids are in bed without simultaneously tending to a screaming baby, but expecting the lifestyle of the average 20something is probably a bit foolish?