Initially I also tried black-and-whiting a bunch of photos I had already, but none really came out ok.
I think the issue is that I thought I was picking things with a high contrast, but this contrast was mostly from large differences in colour (e.g. super bright yellow against a darker green background). When converted to black and white, they were a lot closer in colour than they were initially.
I also noticed everyone elses photos seem to be a different colour of black and white, if that makes sense.
Yeah it does make sense. Professionals etc can actually tweak the tone darkness/lightness of each set of colours in an image (e.g darken all the cyans by 15%, lighten all the reds 20%) individually to get the best contrast mix. That's the secret to getting good black and white reproductions of colour paintings for example.
We're probably all using different filters and some will have presets that increase contrast. My go-to is photoshop express for android; you can really play around with it.