These players love cashing that guaranteed signing bonus but then 2 or 3 years later, they seem to forget all about that huge wad of cash and point to their yearly salary and declare themselves underpaid or worse yet "disrespected".
This is mostly ego for these top players. They all want to be the top paid at their position and many are at some point, but the cap always goes up so then next great at your position who signs a year later will likely get more money. That's just how it works.
Earl Thomas is a selfish *******. How dare Seattle hold him to the contract he signed!