Actually I’ll agree with you that a spreadsheet could do a lot, but that’s a niche solution. Building a good one requires a fair bit of technical know how, and even using one well requires a lot of understanding.

Why would you assume the cycle is regular? It’s a biological process that can vary quite a bit, which is part of why you would want to track it in the first place. There’s also much more to track that just the expected start date of your next cycle. The various tracking apps are quite a bit more involved than just a calendar.