You may well have breezed through maternity leave but this absolutely isn't universal and other people's experiences are every bit as valid as yours
Beleive me, I didn't breeze through it at all, I had very demanding children who didn't sleep through the night and yes, at the time, I thought it was extremely difficult... Until I went back to work and realised that actually however hard it was before it wasn't half as bad for the reasons stated above. If you have a very bad night as a SAHM, you can adjust the following day as needed. You can cancel the meting with friends, you can stay in your pyjamas and you can spend most of the day in front of the TV doing the strict minimum. Just knowing you can do that makes it more bearable even if you cry your eyes out through lack of sleep.
Getting no sleep when you have to get up, do your hair and make up, dress up in heals, face commuting, and then have to pretend that you are alert and really looking forward to deliver that presentation you were hoping to finish that morning because you couldn't concentrate anymore the evening before, and come across as knowledgeable and confident in front of 50+ people, that's horrible stress.
Of course, not all fathers indeed have jobs requiring high level of concentration and some can manage on much less sleep than others. And of course there is a difference between looking after one easy baby and a difficult baby and three other kids.
As others have pointed to though it seems here that every sahm have it hard and people don't understand how so and that it comparable to any job whatever the level. I agree that some fathers should probably help more but in many cases, it is justified that the sahm should do all the night wake up, and indeed many fully agree because they recognise that however much they long for a full night sleep, they need it less than their partner.