To me it's odd to imply people must be stupid because they're expected to read a few simple words such as "Left lane must turn left" rather than to memorize a bunch of symbols.

American signs very frequently combine both styles - for instance, you see the above words next to a sign with arrows for whatever lanes, wordlessly illustrating the turn types permitted.

If someone is a new driver, who hasn't seen the symbolic signs enough, the word signs are a good fallback. The goal is to maximize accessibility.

The system of signs used is an independent question from the level of driver education (which probably is generally lower in the USA than in most European countries though), which also is another question from whether people "are stupid".

The obvious benefit of the European style is that you don't need to speak the language in order to understand the signs, something that is clearly less important given the relative homogeneity of language in the USA.

But at least to me, the European road signs are also more quick to read. For example, the "only right turns" and "speed limit 60" are exactly the same shape and color in the USA, while in Europe there is more of a logical system (which also means you don't "memorize a bunch of symbols", you learn the "language" of the signs), such as blue disc means "mandatory", red circle means "prohibited", red triangle means "warning", etc.