It’s incredibly difficult for a pregnant woman and her toddler, pushchair and changing bag etc to use the tube, so it isn’t a viable option for some people.
@Argeles I was virtually housebound for the first year of my DS's life, due to disability, and when I moved to a flat near a tube station which had a lift, I was absolutely liberated! I was still disabled, and by then I had a toddler in a pushchair, but suddenly was able to get around without hurting my joints!
I agree entirely that some tube stations are just awful, and completely inaccessible for anyone with a pushchair or mobility difficulties, but at many stations it's got a lot better. If you live anywhere near a tube station with a lift, and you want to go somewhere which also has a lift, it's pretty straightforward. TFL has a zoomable map which says which stations are step-free, here: content.tfl.gov.uk/step-free-tube-guide-map.pdf
My DS grew to love the tube with a passion. We had some really special moments on it - including several times when we got on at the end of a line, sat near the front of the tube, and the driver came on, and invited him into the driver cabin for a few minutes before he had to shut the door and drive off without any small children distracting him! (We didn't ask, they invited.) My DS is a teenager now, and still remembers that warmly.
Even an ordinary journey, we read the adverts (I swear it was one of the things that made him such an early reader) he counted the stops, he learned the maps by heart (he LOVED tube maps), he really had a good time. It sounds mad from the perspective of an adult, when a tube journey can be such an exercise in annoyance and grief, but it really did work for him.
Obviously rush hour is never a good idea, but most of the time it's fine. My DS now is very adept at safely navigating public transport as a teenager, and I think a lot of that is due to him spending so much time enjoyably on the tube!