Because it is in an A-5 straight

Perhaps I should have been more specific with how my question was worded. What leads OP to believe that an Ace is _exclusively_ the lowest value card?

It's also in a 10-A straight though (generally; I'm not referring to this particular implementation). It also wins in poker hands that come down to simple high card, highest pair, etc.

Essentially, the A-5 straight is the outlier where A behaves abnormally.

but royal flushes exist in this game. A royal flush is T-J-Q-K-A