Perhaps it's because knowing times tables is a skill we all need in everyday life and it's a lot easier if people can learn them off by heart rather than have to go through the whole process in their head?
That's what the government thinks. Send an email to Nicky Morgan.
(Although too much stress on times tables is bad; I remember crying in Year 4 since I only got 87/90 in one of the routine times tables test handed out).
It's still not as bad as the BBC headline in the website on the homepage which states:
Pupils 'must know times tables by 1…
and cuts off the other '1'.
I do agree that it's got nothing to do with maths. You can just learn them like pieces of vocabulary.