Because you either keep reset the DTC which is active and the ECU will turn off the dash check engine light. The DC will remain in the permanent file are of your ECU and cycle through until the cause of the DTC was corrected. After 1-several complete engine cycles if the problem was not correct the check engine light is then back on. This all means that you are not finding the cause and are guessing at what might be the faulty part. By doing that type of hit and miss more times than not causes other unresolved issues making the original cause more challenging to find.
It may help to find all the permeant DTC as well go look at the I/M monitors.

Your posting is not uncommon for DIY attempting to diagnose problem related to the ECU operation and thinking that that one DTC and th related information to that DTC must be the problem. Most all DTC have several causes and possible parts that could have caused the DTC that each part must be first checked to verify they are faulty or failed. If none are found bad, then the cause must be diagnosed by other causes that could have shown that single or multiple DTCs. One faster method is if the scanner has and the operator has trained experience can view the live data feed or what I prefer is using a oscilloscope and look at real time operations which can faster lead to causes of the problem and the DTC being generated.

 

Example DTC PO171
Which one does anyone think it was caused from.

This is not a trick question, but it is a point of example

Common Causes

The P0171 code can be triggered by several issues, most commonly:

• Vacuum leaks: Cracked or loose hoses, intake manifold leaks, or brake booster vacuum leaks can allow unmetered air into the engine.
  
  2
• Faulty Mass Air Flow (MAF) sensor: A dirty or malfunctioning MAF sensor can misreport airflow, causing the engine to run lean.
  
  2
• Oxygen (O2) sensor issues: A failing O2 sensor may incorrectly report the air/fuel mixture to the PCM.
  
  1
• Fuel delivery problems: Clogged fuel filters, weak fuel pumps, or malfunctioning fuel injectors can reduce fuel supply.
  
  2
• Air/Fuel Ratio (AFR) sensor issues: On newer Mazdas, the AFR sensor monitors the mixture and can trigger the code if it malfunctions

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So to close this out (again, for good), I changed the EGR Solenoid the following day (from my previous post) and replaced the vac line (connects EGR solenoid to MAP). It's been a few weeks and about 500 miles of mixed driving. No codes, all monitors complete.