The last shop vac I took apart had a 2 h.p. rated motor in it.  The motor looked very similar to a motor found in a kenmore magicord canister or in many different brands of vacuums.  The motors in most vacuums are all pretty much the same.  Usually made by Lamb/Ametek- Thank-you. They do a great job with that.  And why would one want to build their own motors.

Hoover used to be an exception and had the patents and manufacturing on the motors in their products.  I have no idea anymore what the inside of a 'made in the last 20 years' Hoover vacuum looks like.  

Yes, most shop vacs are all plastic.  Again this is again to protect from electrocution if the electric parts get wet.

Shop vacs are not recommended for interior use because they have terrible filtration and are noisy.  Again, their intended use is for bulky jobs. They also are the most practical because it isn't figured they will be used on a weekly basis.  

I've also had to replace more than one defective switch on a shop vac.  Again, they put in cheap parts because they figure you'll only use it three times a year.

It's good to have for the times when you really need it like in an emergency. 

I wonder if one could dare put in a drop or two of fine sewing machine oil in the bearing?