The reason that they don't is that they're trying to mitigate their chances of have customers send spam via their mail system.
The more emails a customer is allowed to send the higher the chance they'll use it without properly conforming to anti-spam legislation.
If a salesforce mail server ends up on a spam blacklist that's a bad thing as it's going to block internal workflow emails to actual users across multiple org.
One last note: they can raise this limit if you convince the relevant project manager you have a legitimate use that's not well served by a partner. If this is for something that's not marketing blasts you may want to open a case and pursue this.