They became open world games basically. Collectathons were the precursor and were about as expensive to make. So combine a genre with the expenses of triple A games at the time with the added pressure of NEEDING a mascot that appeals to children (Conker is a major exception and it never did the numbers same with American Mcgee's Alice if you're of the few that has played it) and you have yourself a genre that come ps3/Xbox360/Wii is genuinely unsustainable since the money came from lowering dev costs as much as possible, being easy to monetize, and appealing to broader demographics.

Suffice to say it's easier to simply incorporate some platforming into a 3D action game to break up the main gameplay loop of killing unique enemies in a 30+ hour experience than it is to make unique levels and challenges for an 8+ hour experience. All the while giving you the freedom to make the main character a human and more generally appealing to all age groups.