I've had various stomach issues for a month. went to the er, no tests done. went to my pcp, was prescribed nexium, which i've taken for almost a week. pcp said i had either gastritis, ulcers, or a combination of both. why won't anyone opt for tests?

Tests for what? It is considered improper practice to ever put a gastroscope down anyone's throat without a full few weeks on a PPI like Nexium. It often involves sedation or anesthesia. Insurers rarely pay for it without conservative efforts first. You may want to know immediately but patience is a virtue, especially when most will respond without invasive testing.