I don’t know about you, but 20 minutes sounds like quite a walk. It’s absolutely possible, and can be done for fun, but if your main goal is going to a concert tacking on another 40 minutes walking may be mildly frustrating.
That’s fair, although people often underestimate the time “more convenient” options take. For example, walk 2 minutes to a bus, wait 5 minutes, ride 5 minutes, walk 3 minutes to destination. Not too much faster. Happens all the time with cars in the city. Spend 5 minutes looking for parking that ends up being a 5 minute walk away, and a 10 drive is now a 20 minute transit.
With the amount of rain we get (or just cold or brisk or misty weather in general) for 3/4 of the year, even if the transit option is only 16 minutes (inclusive of the bookend walks, according to Google Maps), it's still infinitely preferable to walking 20 minutes in that weather.
I have a fantastic, well-fitting rain shell, and comfortable waterproof shoes. Rain is absolutely no problem for me, but the general inconvenience of it all (wet exterior, wet face, etc) and temperatures are still enough to make me vastly prefer the transit option compared to a slightly longer walking option.
Another? A 20-minute walk tacking on another 40 minutes to your trip would only be the case if you can drive there in zero minutes, which... I think it's safe to assume is not the case.
And given that we're talking about a concert at some sort of large facility, I wouldn't be surprised if it takes you more than 20 minutes to get out of the parking garage after the event, let alone drive yourself home in the post-concert traffic.
This is also without even addressing how sad it is to me that someone might consider 20 minutes to be "quite a walk". If you're mobility-impaired or there's some severe weather going on, sure, but otherwise... oof.
OP indicated that arriving by car they got there much faster. I assume they wouldn’t have said that if their time finding a parking garage took up a large chunk of time.
Then other person said it would be 20m to walk from the train station. Not 20m from OP’s house.
Assuming the car is generally faster than the train, you now have a choice between train + another 20m walk, or car + whatever time it takes to park.
20m is enough to get the bike out for, but that’s not an option after taking the train.
How long will it take you to get out of the parking garage after the show? I haven't been to Climate Pledge but I've easily spent 15 minutes waiting to leave a garage after a 3000 attendee stage show.