Maybe they do need the money.
After all, Python is a cult, they didn't make a massive amount of money. The records sold OK, but not Led Zeppelinesque standards. The films were more labours of love than commercial money spinners, which might explain Cleese/Idle's poor movie choices for the bucks.
Their last film together was in 1983. Last significant set of gigs (Hollywood Bowl) was in 1980. Last TV series, 1974. And there's five of them, quite a bit to split.
That's a while ago now. Throw in a few bad career moves/business deals/divorces/new family and it soon adds up and the money soon runs out.
If they want to cash in on their success and add to their pensions, then let them.
Sooner them than fucking Michael Mcintyre any day of the week...