The short answer is that only some "bulbs" get pulled down deeper!  And there's a reason why they have evolved that way.  

Just a little about neophytes or “bulbs” before an explanation of contractile roots that pull the “bulb” deeper: Most people use the term “bulb” to refer to underground, fleshy storage organs that contain nutrients to ensure the plants' survival.  The most common types of storage structures are true bulbs (such as the lily, daffodil, and tulip), corms (crocus and gladiolus), tubers (potato and begonia), tuberous roots (sweet potato and dahlia), and rhizomes (canna and iris). 

The best example of "bulbs" that are pulled deeper in the ground are corms, which develop two types of roots.  One is the normal fibrous roots on the bottom that absorb water and nutrients, like so many plants.   The second type are contractile roots which are wrinkled and thicker roots that form as the baby corms are growing. Corms, like all bulbs, have to be planted at a certain depth in order to flourish.  The corms that you plant will bloom (in spring if it is a spring blooming plant) and start to shrivel as new corms develop to replace the mother corm.  These new corms contain the food reserve for the dormant plant to grow again next season. Perhaps you can imagine that as the many new corms form, they can get closer to the top of the soil level which is not where they should be.  So the contractile roots slowly pull the new corms down to the ideal level.

Corms are not the only neophytes that develop roots that pull the "bulb" down.  A few true bulbs, but not all of them, also develop contractile roots such as lilies (Lilium), Daffodil (Narcissus) and Hyacinth (Muscari). 

Courtesy of NYBG Plant Information Service