Though short term capital gains and ordinary income are taxed at the same rate, "other things" apply to ordinary income... social security and medicare taxes, for example. The US tax code is 70,000 pages (or is it 80,000 now?)... anything but "fair"... much of it HORSE SHIT... with special interest carve-outs, sadly.
I am happy to be wrong on this subject. Do you know whether the differentiation exists in Spain or Italy?
In Spain there was a system similar to the US (differentiation between more/less than one year holding) up until 2014 iirc, but currently everything is taxed as short-term gains rates, between 19% and 23% (the rates were lowered a few points when the law changed).
Just going to the PWC website I found this: https://taxsummaries.pwc.com/spain/individual/taxes-on-personal-income
That's right, from 2021 a new bracket has been added for income above 200K, at 26%. Unfortunately tax codes change constantly (usually for the worse).
But are you sure Spain does not tax trading income as business income if volume, frequency etc. surpasses a certain threshold (to be assessed subjectively by the tax authorities)?