While I'm very happy for you receiving job offers all the time (congratulations!), there are definitely reasons for an engineer to take an interview; in particular, falling in love with what a company stands for. The idea that all engineers are mercenaries ready to work for anyone and sell themselves to the highest bidder is an aged one, and one I had hoped was no longer true. If I fell in love with a company (as has happened in the past), I'd certainly be willing to jump through some hoops to "prove" myself to them - if that means going through an interview process so they can fall in love with me as much as I have with them, so be it.
When I see a woman I like, I don't go up and tell her about all the other women that want to sleep with me. Instead, I flirt, I jump through some hoops, make her jump through some of mine, and ultimately establish a strong connection. Or not; that happens too. Same goes for finding work.