Picture from the german wiki article
Being fast on a bicycle has a lot to do with aerodynamics so you go build an aero chassis around a recumbent and bam, fast as fuck boy.
If you do 200 Watts on a regular bicycle on a windless flat and a regular road, you're looking at going about 30kph. With this? 50kph. Obviously generalizing a lot here but you get the point.
They suck ass on hills though, because they weigh ~30kg with the chassis. not so much a problem in motion on the flat, kind of sucks uphill where aero doesn't matter. From my experience you don't see a lot of e-bike variants so far because with EU regulations the motor'd cut out at the speed you'd go for a leisurely ride on a normal bicycle, but maybe it'd be interesting to make them more hillclimbing capable.
Fun fact: in 1968 some dude did 100kph in this with by himself with no outside help.
Chassis are usually some sort carbon fibre or fiberglass and built monocoque, which means you better not ding them babies because repairs are gonna be expensive as fuck.
Prices run around 10k€ and then up to whatever you feel like paying for a new one. But they do look like old timey racecars which is fun. From personal experience of having chatted up a few of the local velomobile riders I feel like there's about a 50:50 split between buying one and somebody going fully "dad in the shed" and just building the entire thing from scratch
yeah, even the other cyclists consider the velomobile people as bicycle cranks