Well, it is complicated!
It is fairly complicated to do so. If you look at a standard W\D-combo, they are fairly tightly packed already.
Now you need to add a compressor to this all. The compressor is quite vibration sensitive, so it has to be mounted to the frame of the machine, but must not block the swinging-area of the tub.
You need 2 heatexchangers with a fairly big surface. Of course, they need to be located on top of the drum so no water enters them, which means the space on top is pretty much used by now.
So the compressor hast to go in the bottom of the machine, which means a pretty complicated pressurized pipeing job.
Then, you need a pretty effective filter to keep these fairly sensitive heatexchangers free of lint.
Because you dry at lower temperatures and the pretty complex air ducting, the fan needs to be a certain bit more powerfull as well.
Now the compressor has to be cooled as well. Another fan.
The condensated water needs to be drained from the cold heat exchanger.
And the thermistor sensing of residual moisture won't work any more either as you heat up to slow and work at to low temperatures. So, you need a nother sensor.
This all has to be wired and grounded, and the electronic control has to keep track of all the additional parts.
For me, this dosen't sound easy at all.
But, it has been done. In Japan, such units are standard. And E-Lux launched a version under the AEG brand here in the EU last year. They start at around 1200€, which is a pretty big pricetag for a washer dryer. Only Miele is more expensive (1600€ upwards).