If you're not particular about what you train in, or the situation, yeah, it's easy to find martial arts instruction. Those people never ask that question because their needs are well met.
If you have previous training, and no instructors/schools about, you can get some limited benefit by solo training (forms, conditioning) or by getting partners to do live drills with and some additional training ideas from video/references. These questions are usually people who haven't had to think about what training without a school or community looks like, and/or what resources are available to them.
If people have no previous training, then they can get some forms and conditioning training, which is of limited value but for some folks, that's fine. As long as it's made absolutely clear to them they will not learn combative value, I don't see anything wrong if people decide they want to learn some forms or do some cardio on their own. (Of course, there's the usual risks of long term cartilage & tendon damage for doing some things wrong, but with a bad instructor or school people do that anyway...)
However, people who have no background in doing resisted training (sparring, active drills, etc.) and want to learn something... well, their only option is to get an instructor or someone who knows enough to practice with.
While I'm generally in agreement with slugster's assessment of the people who ask this question, I don't the group of people confused by media should be deleted - they should have a canonical answer as well. There's a massive amount of misinformation out there, and a canonical answer helps attend to that.