Time makes people forget. Plus there's a little truth in the lake myth, but it gets exaggerated to the point where people think it will prevent something big every time.
Cook county had 24 minutes of lead time on 4/21/67, outstanding for that era especially given that the tornado hadn't previously touched down yet in another county. Oak Lawn had about 25,000 people and the tornado damage path was only a couple hundred yards wide at most, but we got unlucky with a lot of people caught in traffic. I've read the damage survey and it seems like the only F4 damage occurred in Oak Lawn...so if that severe damage would've been more widespread or farther northeast, it probably would've been worse.
There's been some frightening modeled scenarios about what would happen if a violent tornado tracked through Chicago. The area with rather high population density is much bigger though as you said.