Plough's name coming up as a possibility for Oregon State I'll preface this by saying I've never been offered mid-6 or 7 figures to do a job and very likely never will.
That being said, money isn't everything. Yes, $1.2-$2M (the reported range of the OSU coach's salary) is a lot more than the $300-400K that Plough is making. But you can still live quite comfortably on $300K per year. And if it's a job you really like, it's easier to pass up that extra money. It's not like a situation where you're living paycheck to paycheck and get the chance to quadruple your salary.
Most of these coaches are going to jump at the chance to challenge themselves at a bigger program, especially the younger guys like Plough is. But there are always exceptions to the rule and I'm sure there are other guys who could go to bigger programs but choose to stay where they are because they like the lifestyle. And maybe the stress of being the head coach of even a school like Oregon St, which itself is a much different animal from the top programs in the nation, isn't worth the extra money.