All of us probably have, on some level, risky behaviors we participate it. Maybe you're reusing a password between a few of sites, maybe you're driving a little bit too fast sometimes, or maybe you're sitting at your computer a bit more than you should. We take these risks daily, both at home and at work. Risk is a fact of life and of business.
Most of the risks we take turn out OK for us. But some don't. The problem is we tend to ask "why didn't we foresee it" only when something goes wrong, regardless of whether that risk was worth taking at the time. So the question is incredibly biased. Risks don't exist in a vacuum.
Anyway, I'm not an expert on CPUs or the CPU industry, but for what it's worth I'm guessing that they knew that there was a theoretical way to exploit this architecture. Maybe they knew how bad it could be, maybe they didn't. But market forces today are a lot stronger than some theoretical problem that may or may not happen in the future, especially if that theoretical problem would also hurt your competitors. I bet that fixing the problem is expensive in either money or performance. If that's the case, then fixing it when the risk is theoretical could be a bad business decision and might give your competitors an advantage (cheaper or more performant hardware).
I think that a much more interesting question to as is how do we incentivize this industry to avoid these types of problems in the future