That is because fibonacci is used to indicate the ‘guess’timation part. 5 Instead of ‘twice as much work as a 2’ (4). The idea is when you get higher numbers the opposite happens: 21 sounds like a very specific number and not like a rough guess. 20 does.

So it becomes 1 2 3 5 8 13 20 40 inf ?

By that logic shouldn’t people just do 1 2 3 5 10 15 20?