What can I do to appear more approachable? (Context included)

I just found out the two of the most popular guys in my school had major crushes on me throughout high school and sixth form despite this neither one approached me or gave any indication that they liked me.
The first guy I was friendly with an we would have the same classes, friends and would talk often but he never approached me in that way (I had absolutely no idea myself) the other guy apparently had a really big crush on me from high school through to sixth form and most of the sixth form knew (once again not me or my immediate friends) Again both guys were super popular and had not problem talking to girls etc.
What is frustrating about this is that it gives me the impression that I must come across as unapproachable since neither of these guys did anything about it. Also, it is annoying because during my teen years was really insecure about my looks and felt super ugly.. I would notice guys looking at me but always thought it was because I was unattractive.
Anyway since high school a number of other guys have contacted me on my social medias saying they are into me and want to go on dates, again they gave no indication of this whilst at school together.

I know this is super long-winded but my questions are: What can I do to appear more approachable? How was I oblivious to this?