You're going to get more casual inquiries with a custom landing page than an aftermarket parked page where users have to go through the hassle of signing up for an account, putting all types of information on file, etc.
Some people would request a price for curiosity/the hell of it and a registration process is just a barrier. Casual inquiries are opportunities to be converted into sales.
A lot of these parking companies really need to revamp their entire model. It's 2015 and there is not a single parking company that offers optimal customization for a parked page. It's long overdue and I don't know what some industry leaders are thinking by keeping the same boring, washed-up, generic pages for years.