Lilac Spatula - because for lots of people (depending on a lot of variables) the difference between paying a mortgage and renting is still less than the mortgage interest rates.
E.g I pay £312 p/m. The house opposite me, exactly the same design, number of bedrooms etc, rented last week for £675.
25 years of 12x363 = £108,900. Whereas the mortgage interest I will pay is about £30k. So I am already 80k better off buying the house, even assuming rents won't go up over those 25 years, which they obviously will.
Same with someone in a more expensive area - mortgage of £150,000 over 25 years at 3% is £711 p/m repayment and £63k interest. But renting at £1200p/m is £150k more.
Plus, at the end of those 25 years it is very likely I will have an asset worth at least the amount I've paid, and very possibly the amount I've paid plus the interest rates, and even more, which I could then do whatever I wanted with. Whereas if I'd been renting I'd be left with nothing.
Not saying that everyone should want to buy a house - of course everyone has different circumstances. But it's a bit odd suggesting it's financially unsound for everybody.
Also I think people congratulate first time buyers because usually they have saved for the deposit, so its an acknowledgement of that, plus recognition of them achieving a goal. It doesn't mean that goal is one that everyone should aim for, but it's one that they wanted, and have successfully achieved.
I congratulate friends on getting new jobs, new partners, or having another baby. Doesn't mean I personally want those things or even that I privately think it's the best thing for that person, but it would be churlish to say so.