Why Do Americans Have Only One Math...

[ToraToraTora]

.....when we have lots of them?

[Quizmonster]

The earliest recorded use of the abbreviated form ‘math’ in America precedes the earliest recorded use of ‘maths’ here in Britain by twenty years, so they beat us to it! Vive la difference, as the French say.
The only other major school subject ending in 's' and sounding like a plural noun is physics. Americans call it 'physics' with the final ‘s’, just as we do.

[Peter Pedant]

// .... and they say route pronounced rout, except that they say Route 66 like we'd say it.//

yeah but 66 sounz like a mispronounced roman coin -
altho the other week I heard Mary Beard saying sesterce like sess-ter-see

[Peter Pedant]

well when it comes to that

those limeys thought marseille was like Versailles
so put a silent s on the end ( Marseilles in English )
and now we have started pronouncing it !

[Jackdaw33]

The only abbreviations I can remember from schooldays, apart from maths, is Jogga (geography) and Frog (French).

[Jackdaw33]

Reims (French spelling) Rheims (English spelling)

[Quizmonster]

And where did the French get the 'res' in (their version of) London?

[zebo]

Coming into this a bit late, all referendumed out, but in Made In America Bill Bryson talks at length about differences transatlantic and it seems that some are English that we've changed but they've kept eg gotten which annoys many people here. We dropped it yet retained forgotten, begotten.
As far Math(s), I was once told that it is incorrect to pluralise an abbreviation which makes Math correct, not that I've ever used it.