Why don't people like to sell products or services?

All People don't hate selling.  Some do.

But selling poor products to those not wanting them is hard work.

Most people hate hard work that is pointless.

Everyone has to sell at some time, even if it is only every few years to land a job.

Let me expand what Red Fox says.  In a mismanaged company, the owner
thinks the salesperson can compensate for low quality products that
are overpriced, not properly marketed, and offer no advantages over
the competition.  Every product or service offered by a well-managed
operation has a "unique selling proposition" that places it in a class
by itself with essentially little or no competition.  In these cases,
the product or service "sells itself" and the sales staff simply
educates the prospect to the advantages, often leading to a sale, and
making it fun.  What percentage of companies are poorly managed? 
Sadly, most are, and therefore the sales staff is not given the tools
it needs to be successful, making the job stressful and unrewarding.

Big Blue