Why Can Flight And Car Be Booked Separately, But Not As A Package?

I attempted to book a flight and rental car online.  My booking could not be completed.  I called the customer support toll-free number.  The agent attempted to book package, but could not do so.  He told me he would have to book them separately, but would not give me the cost.  The total cost was $67.00 more to book separately. I asked why it couldn't be booked as a package.  He said he would transfer me to a supervisor.  I was then told I would be charged international rates for the call. I ended the call and called the 'toll free' number again.  If the flights and car are available for separate bookings, why can they not be booked as a package as advertised on the website? I believe it is a deliberate attempt to draw customers and then give some lame excuse as to why the booking cannot be completed as a package.  They blame it on the dynamics of the computer system.