Most of them only faced Unelectable type of Opposition party and or candidates for the bulk of their political career. Hence it creates the illusion that they have done well.
Just look at that Daryl David hiding behind Lee Hsien Loong's AMK GRC ward for ages. You think he contributes anything in Parliament? To him, just get paid and continue to milk lah...iron rice bowl mah.
Take him out and put him in a Electable Opposition GRC or SMC contest and that chap will just sink in no time...