Why is the traffic so bad in Seattle Tuesday through Thursday?

One possible explanations is that they are common commute days in a week. The traffic is bad in Seattle attributes to several factors: it's a major metropolitan city with almost 2 million people in the area; its geographic features that prevent building roads more extensively esp connecting segments/patches across lakes and rivers; big companies such as Boeing, Microsoft, Amazon have big footprints in Seattle and the locations of their offices spread the area can also force people to commute around in workdays.