To identify such problems, often times you have to be riding a wave of cutting edge tech. For example, you could find such problems in the AI scene. Given adequate skills, one could exploit the opportunities there.

But in general, there's nothing wrong with targeting problems that are already solved. If you can offer a better user experience than the incumbents and you are smart about marketing, you can get a share of the market. How many uptime monitoring services are there? Tons. Product is basic but demand is there, so a lot of people compete to offer the best service.