Because a café supplies three things a home desk does not: other people in the room, a reason to start, and an end to the day. Remote and freelance work removed the commute and the office, and with them most of the structure that made the day begin and stop. Walking somewhere, ordering a coffee and sitting down among strangers puts a fair amount of that back for the price of a flat white.
The company, without the conversation
The thing people are usually describing is company they are not obliged to talk to. Working alone at home is quiet in a way that gets heavy by the second week, and a video call is not a substitute for it. A room with other people in it, none of whom will ask you anything, is a specific and quite hard-to-find condition, and a café is the cheapest place to buy it.
The commitment device
Having gone somewhere and bought something, most people find it much harder to spend the next two hours reorganising a folder. The money is trivial and the effect is not: you have made a small public commitment to working, and there is no washing machine to unload. A two-hour session in a café routinely beats a five-hour one at a kitchen table, and everyone who does it knows it.
The time limit is part of the mechanism rather than a drawback. Your laptop has a battery, the drink runs out, and the room fills up at lunch. A session with a natural end gets finished; one with no end gets extended.
The noise is the point, up to a point
Moderate background noise genuinely does help some kinds of work — the diffuse, generative kind, where you are turning something over rather than holding six things in your head at once. It is the wrong environment for anything that needs real concentration, and for anything confidential, and for calls. Most people who work in cafés are doing email, writing, and admin there, and taking the hard hour somewhere else.
And where it stops working
The honest limit: a café is a good second room and a poor only room. There is no second monitor, the chair is wrong for a full day, you cannot leave your laptop to go to the toilet, and the seat you want is taken by 11am. People who work outside the house every day usually end up alternating — café in the morning, a library or a coworking space for the afternoon — which is a different answer from the one the question expects, and a better one.
Related: is it rude to work in a coffee shop, and which coffee shop is best for working on a laptop.