Why date someone secret for months to just date the next so publicly?

I've noticed in my past, well 3ish relationships that my partner really seemed to enjoy being with me. We would go on dates, spend as much time as we possibly could together, taking cute pictures and playing games together and etc. However they wouldn't let me post anything about them on social media, I enjoy doing that with my partner.

We would be together for months and the stance wouldn't shift at all. Then something happens, we break up and they move on. Then they post all the time about their next partner.

I can't seem to wrap my head around why this seems to keep happening. Are they ashamed to be with me? Am I ugly but good to be around? Why do I keep being the secret boyfriend?
Why date someone secret for months to just date the next so publicly?
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