I would say we disagree with the premise of the question. Mathematics is not a singular noun. It's a plural. It's the field of all mathematics. Therefore you preserve the "s" because you abbreviate the singular and re-pluralise it.
So somebody in the UK might (not commonally) say "it's a math(matic) concept", but more likely to say "it's a concept from math(ematic)s" or "it's a mathematical concept".
That’s interesting. What about talking about it as a subject or a class? Would you say maths are my favorite subject(s?) in school? Maths are my favorite class?
To the best of my understanding, mathematics isn't referencing a singular object but is used as an encompassing term to refer to content from multiple schools of mathematics e.g. geometry, statistics, calculus, algebra etc. Or in other words, all the subjects covered in math/maths class! 😊
There's usually a long explanation for these types of things which most of the time boils down to "because that's how people have been saying it and it's become the norm."
Many linguistical mistakes have been overused to the point of them changing their meaning. Take "decimation". It used to mean to kill 1 in every 10. Because it sounds cool and has been used in a lot of media, it now mainly means to kill or destroy a large part of something.
The meaning of “siege” has gone from sitting outside a castle or town until everyone starves to just about any kind of military action involving a building. Probably partly because the Iran embassy Siege off the 80s was endlessly represented on TV by footage of the SAS breaking the siege by abseiling through the windows.