Are single page websites bad for SEO? It depends on the competition. 10 years ago you could get away with it. In this day and age it’s almost certain that there’s a 20 page, content rich competitor chomping and your heels.
If you have a one page website and it’s not ranking well, chances are it’s because you have a one page website. A one page website is a bad idea because each page of a website should be designed to rank for a single keyword. If I want to rank in Google for “SEO + area” then my best chance is to create a specific page targeting that keyword. I can’t have the mindset that a single page on my website must rank for 30 different keywords. It’s for this very reason that I always steer my clients away from taking this route.
A situation where a single page website MIGHT work is when there is little to no competition. For example if I was to make up a word that has never been used before and made a website containing that word, then my one page website will suffice. In fact having a one page site with only one paragraph would be enough in that circumstance.
What about owning 50 single page websites dedicated to ranking for a single keyword each? The answer is most definitely not! If a one page website is enough to rank for a keyword, then save your money and just try and rank for those 50 keywords on the one site. You’ll save time and money not having to pay for 50 domains, not to mention the domain renewal fees.
I hope this answer your questions.