You will need time to buy that gpu, upgrade your PC, realise that it doesn't fit into your existing case and that you need a better power supply. Then some time

You will need time to buy that gpu, upgrade your PC, realise that it doesn't fit into your existing case and that you need a better power supply. Then some time to find a better place to be than sitting next to that really loud and hot PC. And it all takes optimistically days, while you can start computing on EC2 within in an hour. Plus it doesn't scale, so you cannot just speed up the results by paying some more. You may need to wait a month (and then realise the param was wrong and you need to redo it) Surely there are some cases when you are better off computing on your own GPU, but I think in all of them you assign pretty low value to the time spent.