The problem is not the actual dev time of a serious side project like a SaaS. Even if it just takes you three months to get to some sellable product with payment integration, etc.--the much bigger problem is that you need multiple shots to get it right (find the right market niche, the right product, the right angle, the right Marketing, the right growth channels, etc.). Then, after more than just three months and many attempts + pivots you just don't have the energy/time/money to keep up. Of course you can clone some proven model but then you face lots of competition which doesn't make success more probable.
People who are succesful did 99 mediocre side projects before they hit the jackpot. But it still took them tons of time and energy. Very often a overlooked by-product which was developed over night and benefitted from all prior experiences will be the cash cow while the main project which took 12 months still hasn't won any users.
Most stop at some point because it's just tough, especially the mental part.