I would assume mostly for convenience, since it is likely easier to rack onto fruit than dump fruit into a primary vessel. Also, people are likely combining secondary fermentation with secondary for purposes of clarity. You would leave some trub behind (which also includes yeast) but there would be plenty of yeast in suspension for the secondary fermentation. I have spoken with a yeast specialist that described yeast in suspension as more active and "better than the lazy yeast in the trub" (his words), so perhaps there is an advantage to using only the suspended yeast.
There's racking to a secondary vessel, that is, moving the wort off of trub ostensibly for purposes of clearing; a somewhat controversial practice many regard as unnecessary as well as risky as it increases oxygenation and risk of contamination and there is a secondary fermentation; which simply means more sugars added for the purposes of fermentation after initial fermentation has subsided or slowed (like fruit, honey or usually something other than malt). The two practices can be combined but they are two different things.