For positive reviews, a business will figure out customers who they already know had a positive experience (quick delivery, continuous usage, etc) and only send them invites to review. This is perfectly legal and the fundamental business model of many review websites - selling the ability to push invites and “manage” reviews.
For negative reviews - no business wants these, and customers with bad experiences are likely to post them by themselves.
What gets left out is the average experience because reviews are essentially cherry picked from the head and tail ends of the normal curve of experiences. This doesn’t render reviews useless, of course. Having a large number of positive reviews is still a positive signal but it is nowhere close to free from manipulation.