There are several reasons why treadmill decks are made with MDF.
FIrst, MDF won't shrink or expand along it's width. An equivalent
solid wood deck would expand along the width and push out the side
rails of your treadmill. Likewise it would shrink and fall off the
support brackets. MDF also flexes when you run on it. It's much
easier on your knees. My current treadmill has a method for adjusting
how much the MDF flexes. The last reason I can think of is that MDF
is smooth. Treadmill decks are waxed to reduce friction and wear. I
break mine down and wax it every 6 months.

Because I run on a treadmill at 9 - 9.2 MPH and put about 20 miles per
week on it, I've worn out plenty of consumer models. I've had plenty
of talks with the repair guy on how to ruggedize one. He finally sold
me on the idea of going commercial. Commercial models still have MDF
decks. I'd advise against swapping it with a solid. If by chance you
do it anyway then let me know how it turns out over time.