Why isn’t philosophical skepticism a valid worldview?
It’s incoherent. If a skeptic doubts everything, they must doubt themselves, their own reason, and their own claim that nothing is certain. A consistent skeptic should doubt their belief that truth isn't discoverable - so they can't make a dogmatic claim that truth isn't discoverable.
Any fact used by a skeptic to make their case must be invalid. Facts imply knowledge of the truth, which the skeptic says we don’t possess. Therefore, a skeptic claiming the truth contradicts himself or herself.
Skepticism is hypocritical. To support their position, skeptics must utilize a double standard, doubting the truth of all other opinions but not their own.