Just Google “electric road bikes” and you’ll see that there are loads:

https://www.cyclingnews.com/features/best-road-ebikes/

They’re more popular in downhill mountain biking because the sport is going down a hill, getting back the hill is just an inconvenience to be mitigated as much as possible!

While obviously it’s handy in commuting, so they’re common on folding bikes, but road bikes are designed pretty much for racing, so there’s less demands for electric versions (but obviously still enough that they exist!)