Old code is everywhere. I once worked at a health tech startup that was integrating with a very successful existing company. All their code was in Turbo Pascal. We were shocked. The funny thing is that their code worked great and even had a debugger. Our code was ultra modern typescript micro services and we debugged with prints and logs. Their product looked dated but was superior in every other way. My guess here is that XLS is a simpler format and the devs found an NPM package that could handle XLS but not the newer formats.