One of many reasons, but a simple takeaway: When you spend your money on something, do you have a rulebook that tells you what is allowed and is not, and an army of people who's job it is to examine what you purchased and then accept or deny your purchase?
If people spend their own money on HC goods and services, they make the decisions and no one's there to question them. An army of non-productive labor and resources is therefore spent elsewhere in the economy, guaranteed to be spent more productively.
Further, the theory is that if one is spending their own finite resources for their benefit, they'll be much more careful and efficient with that spending, lest they waste it and it runs out.
The incentive with the HSA is that 100% is covered up to the deductible. Co-insurance starts when well, insurance starts paying. So if one can spend wisely, even build up a balance over years well in excess of the annual deductible grant, a person starting young (when they need little to no services) might never invoke insurance payment for healthcare ever in their entire life.
There are plenty of people that could be like this. People who make all the way through with only visiting the doc for relatively minor stuff -- colds, infections, fractures, etc. -- acute minor problems. No heart disease, diabetes, or any other major chronic condition.
That's most people.
Right now, every time they see the doctor, get a prescription, etc., it triggers an entire chain of processing that involves many people at both the provider, and the insurer. That costs.