I use my own text editor too, written using my own programming language. Fortunately Operating Systems suit my needs and I won't have to write my own OS ;-)
Compilers have a huge advantage over other programs: they are fully deterministic since they depend only on input files, command line arguments and few environment variables. It makes bugs easier to reproduce and fix compared to interactive applications, programs with networking, multi-threading...
Pretty sure most modern compilers are multithreaded, and do exhibit a slew of practical nondeterminisms, which is how/why projects like Reproducible Builds were formed.
In general, most compilers are generally single-threaded for most of the compilation process--at the very least, compiling a single file (translation unit) is almost always done using just one thread.
However, nondeterminism does creep in in various places in the compiler. Sorting an array by pointer value is an easy way to get nondeterminism. But the most common nondeterminism in a build system comes not from the compiler but the filesystem--"for file in directory" usually sorts the file by inode, which is effectively nondeterministic across different computers.
A possible reason is the use of Static Single Assignment (SSA). While it makes many optimizations easier, it has to translate its IR out-of-SSA to generate code. This is very expensive as it needs computation of dominance on the Control Flow Graph, insert copies and much more. But it's just a guess.
