Is mental arithmetic practice not part of a general math education curriculum?
You'll be able to improve your mental arithmetic performance by orders of magnitude from pure memorization + practice alone, but this is not how people typically learn math in general, and rote memorization will not make you a good mathematician in the long run (it's probably not even good enough to get you past university-level exams).
I too am a maths graduate.
My peak mental arithmetic was about age 11 by then I had learned times tables etc by rote and could deal with money - being the UK this was LSD so had to convert between 20 and 12s as well as decimal. Then in the next few years we want decimal so no need for that daily task of converting and we got calculators if we needed figures so little practice - I could still keep track of what the total amount I would pay in a supermarket and calculate change quicker than the assistant using the till but even that has gone as you just pay by card.
Nowadays do you need to be good at mental arithmetic, probably for doing quick estimates but I don't know if educators think that getting mental arithmetic working well is needed.