Public transport can be an absolute pig for many reasons.
Walking to the bus stop.
Standing around waiting for a bus.
People shoving their way in front of you to get on the bus.
Standing on the bus trying to not get sent flying.
Getting through people who just won't move so you can get off the bus.
Making your way through crowds of people, crossing multiple pavements and roads to get to the train station.
Getting through the barriers at the train station.
Walking down (or up) loads of steps to get to the platform - lifts and escalators are frequently out of action or are in completely different directions to where the passenger flow is directed.
Standing around waiting for the train.
People shoving in front to get onto the train.
Lifting oneself and luggage onto the train.
Standing on the train (often whilst other people's massive cases have seats and aisles to themselves)
Getting down off the train onto the platform with luggage.
Going up or down steps from the platform with luggage (see earlier comment about lifts and escalators).
Getting through barriers with luggage and people shoving in front.
Finding way to tube station that may or may not be easily accessible.
Waiting on platform.
Getting on tube with luggage and no seats.
Getting off at correct station with luggage. Platforms frequently have massive bloody leaps down from the train.
Making way out of tube station, lifts or escalators not guaranteed, weird, confusing routes not well signposted.
Possibly having to find another part of the same tube station for an ongoing tube or travelling to the connecting rail station.
Repeating attempt to find correct part of rail station, steps,platforms, trains.
Getting off again.
Making way out to buses.
Catching buses.
Walking to destination.
Add to that extra difficulties, such as
Cancellations
Weather
Large crowds
Unfamiliarity with area
and age/health related ones, such as
Back pain
Hip/knee pain
Shoulder pain
Lack of peripheral vision (maybe eyesight is deteriorating) making it hard to see directions/signs/adjusting to the people barging their way in front or judge the leap down from the train onto the platform with luggage.
Hearing issues making it hard to hear announcements or know quite what's going on
Being conscious of being smaller and less agile than the people around
Stiffness in joints meaning sitting/standing/walking quickly and securely is unpleasant
Lack of toileting facilities
Just being tired and weaker physically
Compared to a cab to an airport, pretty much level surface all the way, assistance more available (rail assist is far more unreliable when you're actually reliant upon it, as any search of X/Twitter will demonstrate), a guaranteed seat for the majority of the journey and absolute priority transport services?
I know which one is more tolerable - and it's not the one that gets her to your front door.