I'd say that's because we don't understand what we mean by "understand".

Hardware that accurately performs maths faster than all of humanity combined is so cheap as to be disposable, but I've yet to see anyone claim that a Pi Zero has "understanding" of anything.

An LLM can display the viva voce approach that Turing suggested[0], and do it well. Ironically for all those now talking about "stochastic parrots", the passage reads:

"""… The game (with the player B omitted) is frequently used in practice under the name of viva voce to discover whether some one really understands something or has ‘learnt it parrot fashion’. …"

Showing that not much has changed on the philosophy of this topic since it was invented.

[0] https://academic.oup.com/mind/article/LIX/236/433/986238