For one thing, only some very specific animal species use tools, and then in specific ways that you need to observe closely to notice.
But also, what makes you believe that our ancestors didn't notice? In ecosystems where they observed that, it might have been so routine and obvious to them that it was not something even worth noting. After all, historically, humans have treated animals similar to people for a very long time (e.g. putting them on trial for theft and murder). When you already have that mindset, a tool-using animal is very unremarkable.
But also, what makes you believe that our ancestors didn't notice? In ecosystems where they observed that, it might have been so routine and obvious to them that it was not something even worth noting. After all, historically, humans have treated animals similar to people for a very long time (e.g. putting them on trial for theft and murder). When you already have that mindset, a tool-using animal is very unremarkable.