Why would he like my pics but never message me?

We went to high school together for like two weeks in 2015,… he would often like my pics i’d see his name pop up every now and then but NEVER message me for the last year or so, i secretly sometimes hoped he would message me. I didn’t want to do it first then in case he just likes lots of girls pics and i would seem desperate for thinking it meant he wanted to talk.
I uploaded some pics recently and he followed me on instagram that was in my bio, so I decided to message him telling him I remember him from school! We have been talking for a week or so and we really vibe he said he was surprised and flattered I remembered him and even messaged him first
why didn’t he make the first move if he was interested?
Why would he like my pics but never message me?
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