Because photograhy and videography has a lot of things in common - similar concepts of composition, similar technologies, similar people who are interested in the topic. Its only logical to have cameras that can do both - and its obviously possible to build these.
Its also happening for more than a decade and Canon themself introduced the video topic to photo-cameras with the (back than) groundbreaking (!) Canon 5D Mark II. It offered Full-HD recording on a fullframe - something completely unimaginable at the time. (This was especialy crazy when magic lantern added FHD RAW to it). All of this was achieved while the 5D was still the most popular and pretty perfectly rounded photography camera at the market. Its untill today one of the most broadly used Full-Frame DSLRs.
There is no reason to not implement full-grown video features to a photo camera. While SDI, XLR, V-Mount etc may be nice accessoires - all of these can be worked around.
Especialy small companies or solo-video/photo guys dont want to buy 2 video AND 2 photo cameras - this way they can stick to just 2 cameras which can do both (nearly) equally well.
Canon did the market segmentation-game quite some time, with the (for videographers) rather disapointing Canon 5D IV and EOS R cameras and it wasnt realy appreciated by the video/photo-guys.
1) Well, technically Nikon had the first “HDSLR”, the 5D/II is just the one that caught on.
2) No they can’t. Maybe if you just mean nothing but how the image looks in/under certain/the right circumstance, but otherwise, no a DSLR/MILC is not going to do the job “(nearly) equally well”. Do they have their uses? Yes. But that’s not the same as the general sense that we seem to be talking about.