There are several reasons:
* Germanic
WIDO is a frequent name element (both in productivity and in usage) including Witegis,
King of the Ostrogoths. In contrast,
WIT is a rare name element occurring mainly in the name
Alois.
* As you see, the word guide has preserved a final "d" sound but
Guy hasn't; suggesting the alternate etymology
* Finally, you see people occuring as
Guy in Old French sources and Wito in old
German sources and
Vitus in latinized spelling. This argument is the most compelling.