The reasons above posted aren't the only reason(s).
A lot of states require a physician, PA or a nurse to also sign the DNR for it to be valid. Other states require the person to have a terminal illness. So wherever you are may have built in restrictions on whether or not a DNR would even be valid to begin with. It is also inaccurate to say a suicide attempt "invalidates" DNRs universally, although a lot of states do. Some don't, take NY as an example, out of hospital DNRs place patient autonomy and the right to refuse resuscitation ahead of all other competing medical trauma including suicide and possibly even homicide situations (NewYork State Department of Health EMS FAQ: DNR).
Rather, it is moreso because someone who is either potentially murdered or commiting suicide who magically has a DNR it will be taken at face value to not likely be true or unverifiable in the hot moment an EMT arrives so EMTs will then pass the responsibility onto the hospital and try to save the person on the assumption it is not binding, not because it is invalid but because they don't want to take the risk of getting sued for failing to care for someone. Think about it in terms of also EMTs assuming possibly it was murder not suicide and it was being set up for a payout. They just have no clue and have one goal, to save someone in an emergency. Leave the paperwork for a hospital to worry about, I can see EMTs thinking.
Even with this said, though t
here are still current ethical and legal dilmenas as to whether ER staff should or shouldn't honor DNRs from suicide attempts which signals to me that it is not a categorical, automatic void. If it was a universal truth then there wouldn't be an ongoing debates around it. Right? Maybe, not sure.
With all this said though, I strongly suspect most EMTs nationwide in America are likely to let the hospital take the risk of not saving a life than they will take for their company, especially in light of suspicious circumstances, coupled with state laws varying it is still going to depend. I don't personally think it is too likely an EMT will withold care in a traumatic situation for whatever that is worth though.