I think it's both of those reasons. Collecting commission on one 10-year mortgage, or five 2-year mortgages... I doubt they get 5x for the longer term, if anything.
I'd like to understand how people think about a longer-term fixed deals. How do you know where you'll be in more than 5 years? If you need to move don't you get hammered by the ERCs? Can you really rely on transferring products?
Habito One is an interesting product, that seems to be a lifetime fix (Which I have personally never seen before the UK but maybe it exists) with no ERC! Obviously the rate is less attractive but rates are still so low and I think you can offset as well.