Why can't light rail use the same rollingstock and tracks as heavy rail?

i've always wondered this

lightrail, streetcars, trams. Why must they be built to be incompatible with heavy rail? why can't heavy rail be built with a bunch of level crossings, street level stations, slow speeds, and function exactly the same as light rail?

if compatible, the rail could act heavy at designated tracks and light at others, removing what would otherwise be an interchange. it would also allow the light rail to have a higher top speed at express areas.

what am i missing?