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.
It's mainly because of latency. If you're recording in a studio, you'll definitely have a microphone (would be kind of hard without it). XLR microphones have several advantages over USB mics, including: 1. You can use (heaven forbid) firewire; 2. You can bypass the internal preamps (dynamic microphones usually don't even have any, but condensers have a little bit to bring it to a mic level); 3. You can use mixers, effects boards, etc. with them; etc. Most people will use XLR microphones, which are, guess what? Analog! The DAC only occurs from the PC end.
Digital-to-Analog conversion takes time, but Analog-to-Digital conversion takes significantly longer. That's because ADC requires sampling, as well as bit-matching. 16-bit bit depth = hexadecimal-to-binary, and is the smallest. Increasing the bit depth increases ADC time, while increasing sample rate does the same.
So, to answer your question, it's for zero-latency monitoring.
The best sound production, recording and reproduction is based on physical effects, not digital representation, which is an approximation of the physical effect (a sampling).
physical [ fiz-i-kuhl ]
noting or pertaining to the properties of matter and energy
What's more real than that?
And another reason why professional studio monitors use analog signal as opposed to digital, is that even if you do work with computers to do your sound processing, you can choose what DACs and ADCs you want to use and get the intended purest input and output of those DACs and ADCs fed to your analog devices. Otherwise if your studio monitors had DACs in them, their sound profile would be heavily influenced by the frequency response graphs of its DACs + internal amplifier.
Key lesson to take out of this is that, between analog devices, always use analog lines because that removes extra digital processing/approximation!
Between digital devices, if there's no compression involved, no sample rate conversion and no bit depth conversion, you can run any kind of line, if won't make a difference.