Because no one sell enough ip to actually buy a planet. I tell you i've been trying for days and I have a stong clanmate.
An open market would be much more interesting.
Right now, there is no invectives to get involved in PW beside conquering the galaxy.
Here's an idea : XP would only serve for com.
PW factories allow to use the unit or fac they build in a limited number of battle. You can sell those allowance in a marketplace. That's for a starter.
You could also build your fac on somone else's planet in exancge for a share of the production.
That way you don't have to gain a planet, which is hard to spend your $ which in urn give you an invecive o sell your IP and get involved in PW