What technical reasons do builders use white baseboards over wood-woodstained?
Painted white is cheap. That's really all there is to it, no reason you can't do it differently, and you can find plenty of non-white baseboards in the states - but a preponderance of Florida housing is mass-built developments that are going to run towards cheap and standardized.
Wood is common in older houses, but once some idiot decides to paint it, it becomes a huge job to strip it back to the original, and the paint companies ran successful propaganda campaigns (which had the delightful effect of getting a lot of white lead paint into places it might not have been otherwise) to sell paint in the 30's and 40's