Presumably because "plant" was a word used to describe all sessile, photosynthesizing organisms visible with the naked eye long before we developed sophisticated enough tools to scientifically separate sea-weed from other plants.

Also, I'm not an expert, and I know Wikipedia isn't the most authoritative source, but https://en.wikipedia.org/wiki/Plant#Alternative_concepts seems to suggest that green algae are, at least at times, scientifically considered the parent group of land plants. So there's that, too.