[lekanwang]

You'll need more patients, which is expensive.

But yes, you're actually right -- sponsors do run stratified phase 3 trials where subsets of patients are measured independently per pre-approved statistical analysis plan. The rub is that your patient population and your trial design needs to be sufficiently statistically powered to do that kind of analysis. The required number of patients quickly increases for the number of stratifications/subsets you're trying to prove an indication for, and patients are extremely expensive, so sponsors generally try to run the smallest trial possible while still being adequately statistically powered.

This is generally why in Phase IIs, sponsors will stratify by exploratory biomarkers, where there may not be sufficient statistical power per stratification, but can give them some idea for what to include in the IIIs.