Why don't the major drug companies prevent generic drug competion by
lowering the price of the drug prior to the patent expiration?

Because most people are too stupid to realize they can get the same
drug at a cheaper price, insisting instead that they get the name
brand instead of the generic.

The other commenter is partly right, but there have been occasions
when they did exactly what you said - try and freeze out competition
by making it not worthwhile. But you only have to look at the home
painkiller market to see why they mostly don't - branded products can
be 4 or 5 times the price of exact generic equivalents, but people
still buy them by the bucketload.  Even many supposedly rational
physicians react in much the same way.  Also, the big companies don't
let those patents go willingly - clever use of the patent system can
extend brand sales with the use of better/different formulations aimed
at different markets, with different indications.  They aren't in the
business of giving up sales value without a fight!

1.  Original brand products usually have well established customer
loyalty, hence usually those original drug manufacturers have no real
reasons to lower their prices (profit).
2.  In most cases, before patent on particular drug expired, its
manufacturer (patent holder) will manufacture an exact same variant of
the product and market it as "generic" under different brand (in most
cases, the manufacturer will contract a different company (generic) to
help them market the new brand).  This is also a very effective
marketing strategy to reduce lost of market share when patent expires.