Why is Windows 32-bit called Windows x86 and not Windows x32?

x86 is the name of the architecture that it's built to run on (the name comes from a series of old Intel processors, the names of which all ended in 86, The first of which was the 8086). Although x86 was originally a 16-bit architecture, the version in use today is the 32-bit extension.

x64 is actually more correctly "x86-64"--the 64-bit extension of x86. It was developed by AMD under a license from Intel, which may account for the difference in nomenclature (that and the fact that both architectures have seen simultaneous use for a long time, so marking the distinction is important). All things considered, though, it's easier to just say x64, though, so we stuck with that.

Intel CPUs at the heart of the PC architecture used to have model numbers ending with number 86. There was the 8086, 80286, 80386, 80486, 80586 (the original Pentium), 80686 (Pentium Pro)... Applications and operating systems (DOS, Windows) able to run on one of these CPUs would most likely run on another CPU of that line, albeit slower or faster. There also used to be competing CPU architectures from other manufacturers, incompatible with Intel, such as SPARC, MIPS, ARM, Alpha. When comparing architectures, people would use the x86 moniker to refer to the Intel main line of 16/32-bits CPUs. So you would have the x86 version of an application, the Alpha version of an application, or the SPARC version...

Then marketing concerns took precedence over engineering and Intel started to give its CPUs non-numeric names (Pentium2, Core2) and the technical model number was buried in the tech specs.

When AMD came out with the 64 bits extensions of the classic Intel CPU instruction set, applications and OSes needed to be recompiled to take advantage of the new features, making them incompatible with the previous Intel CPU models. AMD originally used the AMD64 moniker for this new platform, but then Intel started making AMD64 compatible chips (as Intel 64), and it was renamed to x64 because it wasn't an AMD exclusivity anymore.

So although both x86 and x64 platforms start with an x, the x doesn't stand for the same thing, one being the CPU model number (286, 386...) and the other being the manufacturer (AMD64, Intel64).