All studio and professional grade monitors in general have analog inputs (TRS/XLR/RCA etc...). High end monitors have the shortest internal cable run to reduce loss of quality yet this is only a tiny fraction of the total cable run from the DAC. Why don't monitors have digital input all the way up to the internal amplifier. Digital-analogue conversion could take place a few centimetres from the driver. Why couldn't Powered monitors have optical input? Optical cables are not more expensive than good quality analog cables and there is no reason DAC's couldn't output SPDIF?
I think the answer is that they're looking to have the cleanest signal path.
The introduction of a DAC is just one more link in the chain, and when the chain is only as strong as the weakest link, the fewer links, the better.
Also, high-end DACs are an industry unto themselves. High-end monitor companies focus on the mechanical properties of the speaker, the clean signal path, etc. To include a DAC is to relinquish part of the signal path to another company.
Mostly because of preference. Most people prefer to be able to choose an amp they like with monitors they like. Keeping the parts separate also makes maintenance easier. There may also be concerns about the power of the amp interfering with the speaker some or vice versa, though I'm not sure if this would really make a significant difference.
The long and short of it though is that powered cable runs don't really matter all that much. You can't pick up much noise after amplification because the signal floor overpowers most ambient occurrences. The short runs of cables inside I think is more a marketing gimmick rather than something that actually matters.
So simply, it doesn't matter that much if it is internal or not, as long as it is a short analog run to the amp and decent enough quality speaker wire is used. It's more convenient and more maintainable to have the amp and speakers separate, so that's what is done.