"I die free"
A mature symbiote must form a close connection between its mind and the mind of its host:
If I remain on this planet, I will eventually be taken over by my mature symbiote.
— Teal'c ("1969", SG1 2x21)
This is the case even when the symbiote is believed to be friendly and is not expected to dominate the host's mind as a Goa'uld, such as when Shaun'auc, a Jaffa, believed that the symbiote in her pouch had converted to the Tok'ra point of view:
Shaun'auc's symbiote is fully mature. It must take a host within the day, two at most.
— Teal'c ("Crossroads", SG1 4x04)
Teal'c, a former slave Jaffa who has pledged to die free...
Tal shakka mel. I die free.
— Teal'c ("The Nox", SG1 1x08)
...is likely unwilling to risk his right to self-determination by joining with a mature symbiote.