One possible reason has to do with the fact that
when a software has more users, this attracts
even more people to purchase it as well because of
the compatibility. If, for example, I know that all my
friends have Word, I will perhaps also get it in order
to exchange documents with them.

In summary, the sales of a software product
1. Decrease with price (it sells less if it is more expensive)
2. Increase with the number of people already owning it.

This means that for Msoft it is well worth it to have
more users, even if some of them don't pay for the
software, because this will cause more legal buyers
to come and purchase the software for the full price.
If you want a more mathematical explanation, let
me know.

So, instead of giving some copies for free, Msoft simply
puts a very loose protection on the software, so that
some people would copy it while others would buy it and pay
the full price.