When litigation fails, why do some patients avoid paying your costs?

I am often asked why MDOs sometimes resolve medical negligence claims in favour of their doctors but on the basis that each party pays its own legal costs.
Why don’t they pursue the patient to cover the unwanted and unasked for legal costs involved when the patient’s allegations have been wrong, possibly even unfair?
After all, medical negligence claims are expensive to run. While the state pays for the judges and the courts’ administration, the parties to these civil disputes have to pay their own legal representatives — unless they receive legal aid.
At a seminar run by an MDO in Sydney back in the 1980s, where speakers were asked to look at alternative ways of resolving claims and compensating injured patients, one eminent lawyer said that it was important that the legal process was expensive because this discouraged unmeritorious claims.