They sometimes do, but some of these older brick houses use lime mortar which is far more breathable. Allows moisture in and out. Some older houses are also not that water tight anyway or often have moisture issues.

If you use the wrong foam, it absorbs that moisture. Bad. Think mouldy sponge. If you use the right foam, but don't ensure ventilation and tackle moisture issues? Still bad. If you don't mix the foam right or use too much? Possible structural damage. Or the foam's flammable which is an issue in an old house with old electrics.

The person you're responding to mentions people doing it themselves with an expanding foam gun, which as someone who worked in the industry gives me the shivers tbh. I mean, maybe they'll get lucky, but I wouldn't buy that house. If they used the wrong foam or did it wrong, it's not as if you're going to be able to remove it or start over. It's there forever now.

He specifically stated he had a cavity. If you don't you can do external cladding insulation and internal insulation.