Several reasons.
Salvia may not affect you that way. The Salvia you've been smoking may not be potent enough.(Is it supposed to? I didn't think it was strong enough to do that) You might not be predisposed to believing such things (fantasy prone personalities are much more likely). You might get visions, but not interpret them that way due to your upbringing and experiences. Etc.
The brain is a bag of complex chemistry, and plants are equally complex chemically and outcomes in combination, while generally likely, are never 100% certain.
Salvia may not affect you that way. The Salvia you've been smoking may not be potent enough.(Is it supposed to? I didn't think it was strong enough to do that) You might not be predisposed to believing such things (fantasy prone personalities are much more likely). You might get visions, but not interpret them that way due to your upbringing and experiences. Etc.
The brain is a bag of complex chemistry, and plants are equally complex chemically and outcomes in combination, while generally likely, are never 100% certain.