The answer is Because his lady contains all the riches within herself.
Sonnet XV is one of the poems from the collection of poems by Edmund Spencer called Amoretti, where he speaks about his love for Elizabeth Boil.
In this sonnet, he asks merchants why should they go so far to look for riches when his love in herself contains all the riches that can be found anywhere:
"Ye tradeful merchants, that with weary toil
do seek most precious things to make your gain,
and both the Indias of their treasures spoil,
what needeth you to seek so far in vain?
For loe my love doth in herself contain
all this world's riches that may far be found, (...)"