Why does X seem to be the abbreviation for everything?

Well, X sorta looks like The Cross (crucifix), and the Greek letters Chi (pronounced Ki, and is written as X) and Rho (pronounced like rho, and written like P) are the first two letters of Christ in Greek and become the Chi-Rho symbol. So X has symbolized both Christ and the Cross for some time, and then it just becomes cross (like pedestrian crossing).
X is also (one of) the most widely used algebraic characters, you use it to signify the unknown in an equation (but also something that does have a specific value), and so it sorta carries over into language (whereas all the other algebra characters sorta stay in math, unless you need characters in addition to x, and then it usually goes x,y,z,n,a,b,c.. but you can do whatever you want, it doesn’t have to be that way).

Xmas and Christ , the X abbreviation is a symbol of the Cross and Crucifiction

You hit it with your last reference @AshlynM. X for the cross.

In days before widespread literacy, a person could sign a legal document by making the symbol of the cross on the paper – a “swearing before Christ” that his sign was legitimate.

These days, it’s used often in medical terminology Rx, Dx, Px etc. The most reasonable explanation I’ve heard for that is that it’s an infrequent letter in regular language and so the abbreviation is instantly recognizable in a medical transcript or patient files.

@Blueroses My favorite medical X is “Sx” meaning “surgery”, because it doesn’t just mean “s + all the other letters”, it also has a nice visual representation of how surgery works…

In mathematics, “x” indicates an unknown or undetermined value.

Another vote for Chi Rho, at least where X is the abbreviation for Christ.

I think the other X abbreviations come from the “X” as the way to sign your name when you don’t know how to write, and “X marks the spot”.

Because people are lazy.

X = 10

Yes I think X was and is used if you don’t know how to sign your name and I think this still goes back to being a symbol of Christ and the Crucifixion