Why don't many people buy Washer/ Dryers? Are they not very good?

They are a compromise. They do both functions but not as well as separate machines.

Just got back from Europe where all our Airbandb's had the "All in one washer/ dryer". Nice machine to run a couple of shirts or undergarments through to freshen up. But worthless as a family laundry machine. Draining the water reservoir often in dry cycle, unable to dry heavier fabrics, (denim for example) and limited capacity! Space obviously is a huge reason to have them. In the US/Canada there generally is enough space in apartments and houses to use separate washer and dryer units.

We don’t ? Lol

If you mean the upper/lower washer/dryer units, those are good for small apartments, dorm rooms or an upstairs bathroom. That'll do for one or two people.

If you have a family in a house and you're doing laundry for adults and kids, going through towels, sheets and clothes you need a bigger set.

My washer is 120V My dryer is 240V. So there is THAT.

to expensive

Combo washer/dryers make sense *only* if space is limited. They sound like a great idea, but there are reasons that people opt to go with separate appliances. Perhaps you've heard the old saying "jack-of-all-trades, master of none?" That appears to apply here. Different models may be a great washer, but poor at drying...or the reverse. When shopping for reliability and performance, it's often easier to find it in appliances dedicated to one task.

There's also the reliability issue. One machine that does both tasks is bound to be more complex (aka more things to break). If something breaks, it's often not cost effective to repair. If that happens with separate appliances, you're looking at replacing just the washer *or* dryer.

Everybody I know has them

most are made in China now .........................