The US has never signed the Vienna convention on Road Signals, which is what is used everywhere in Europe. Neither have Canada and Australia, who mostly follow the US standards on the road-signs.
As mentioned in the comments, US road signs are mostly verbiage rather than pictograms, and there's an implicit expectation of English reading proficiency at some minimum level as part of driving requirements. There may be some historical reasons for that (mostly exclusionary/discriminatory, unsurprisingly), but the fact now is that change is nearly impossible.
In the US the population is very conservative (in a sense that it is averse to change), and is very loudly protesting any change even if it is objectively for the better (e.g.: loud protests against about, well, any change happening in the US). In fact, coming up with a unified standard within the US was a long process that was only completed in the 1970s and required explicit threat of withholding money from the States by the Congress (and still some States have slightly different rules than others). See here.
I mentioned the pedestrian crossing lights in the comment - they used to be "Walk"/"Don't Walk" verbiage, but are now a "Walking person"/"Standing person" pictograms. That change started in 1971 with the introduction of the pictograms in the MUTCD, and was only completed in 2009 with the removal of the text signs from the code. That was just one sign changed.
In addition to Canada and Australia that use similar standard to the US, many other countries didn't sign up to the European convention (many in Africa, South America, China) - see the Wikipedia article. Many of them use a mix European-styled and US-styled signals, but almost all use pictograms (comparison table).