All player build structures let light through… from several sources but most importantly the sun itself. During day my carefully made dungeons are illuminated… making lights completely useless to place. PVP or PVE I would always want to have my surroundings at least slightly realistic. And walls do not let light through!!
If this is way to annoying to walk in the dark in your own base even during the day… Make it a server setting. Or moddable (I am looking into that). I know it can be changed as in early access two layers of foundation stopped all light. But even that is not enough anymore on live now.
Desperate basebuilder here… my dungeins need their spooky darkness even during the daytime.