Here's why:

* Sieve tube elements, the main conducting cells of phloem, are anucleate (lacking a nucleus). They are specialized for efficient transport of sugars and other nutrients.

* Companion cells, which are connected to sieve tube elements, retain their nucleus and other organelles. They are metabolically active and support the sieve tube elements by providing them with energy and other essential functions.

So, while the sieve tube elements are the primary transport units, the companion cells are their essential partners, providing the necessary support for their function.