I have written transpilers and would not want to use the term compiler because it would be a deceptive description of the work involved. It would be a pretentious aggrandisement akin to saying I built a car because I changed its air freshener.

A production ready optimising compiler is one of the greatest achievements in software engineering. A level of complexity and sophistication few of us can reach. A transpiler is often just a trivial text transformation that a junior programmer can do. The purpose may seem superficially similar but the effort and expertise is so totally incomparable that a specific term is justified.

edit: it reminds me of the difference between a ship and boat. A transpiler will use some-else's compiler/interpreter but a compiler won't use someone else's transpiler. The two terms are meaningful to those intimate enough to need to distinguish degrees of sophistication but seem redundant to those who merely use them.