The cause of medical negligence cases is medical negligence. To be sure, the legal system does not do a very good job of holding careless health care providers accountable (far more people are injured or killed by medical negligence than ever bring a lawsuit much less win a lawsuit) but the legal system does not cause medical negligence.
Here is some interesting data regarding drug errors by some people who are actually working to prevent patient harm. Note that the information is from U.S. Pharmacopeia, Center for the Advancement of Patient Safety (CAPS), a group in the health field, not a bunch of lawyers.
The group found inter alia that during the three-year period of 2001-2003, there were 532,144 medication error records submitted to MEDMARX, a voluntary reporting service. Approximately 4.4% of these (n= 23,689) cited Wrong patient as a Type of Error. Nearly 50% of Wrong patient errors reached the patient but did not result in harm (Categories C and D) (Table 1.) Although only 1.3% of all Wrong patient errors resulted in harm, there were 10 sentinel events (including 3 fatalities) associated with this Type of Error. The number of fatalities from other types of errors is not a part of this report. The amount of medical expenses associated with errors and harm causing errors is not reported.