Because the Goa'uld and the Tok'Ra were among the few races in this galaxy and the next that didn't even attempt to create any cloning technology, mostly because, as you say, as long as they have a new host every few hundred years, they have an indefinite lifespan.
Cloning in Stargate was attempted by numerous races, but even the most advanced, the Asgard, could not perfect the process. The only race ever to successfully clone a symbiote separately from its host was us, the Tau'Ri, but even then we don't know how long it would have survived, because it was destroyed shortly afterwards.
Ba'al was the only symbiote to ever clone itself along with its host, and that was only because he got ahold of some Asgard cloning technology.
In addition to the technological limitations, there was also the matter of pride. The Jaffa especially wanted to rid themselves of their dependancy on symbiotes, hence the introduction of Tretonin, and humans themselves, as Jack and Daniel stated, would not be particularly open-minded when it came to having a "Snake in their heads" (Most of them, anyway). So the issue of not having enough willing hosts was also a concern for the Tok'Ra to consider, as a symbiote cannot survive without a host body.