The GP comment gave a reason: some websites might misbehave with longer passwords, so it's not actually free.
In my opinion, it is more reasonable to assume that most websites will behave correctly with even longer passwords, and solve the odd misbehaving ones on a case by case basis.
I haven't yet, in my many years of "being on the web" encountered a single website with the truncated password problem described above, so for me, it's a weird statistical anomaly to be ignored.
If you have been burned repeatedly by some such websites, you will have a different outlook, and will generate your own passwords accordingly.