Why are offline custom races so completely abysmal?

Why aren’t all the custom race options from online available offline? Why can’t I choose AI cars? Why can’t I make my own playlist? Why have these options mostly been available online but not offline?

Please see here::

http://forums.forza.net/turn10_postst40929_We-can-t-choose-cars-for-the-Drivatars--Really.aspx