Why don't the airlines make mandatory for every passenger to have a covid-19 test?

so that they can sure safety to their passagers .

The cost may be one thing. The big factor is the turn-around time which is too long. This makes it impractical for airlines to do COVID-19 tests before boarding.

TESTS cost money and take TIME to process.

Testing is getting better. There is NO GUARANTEED 100 % accurate test.

You can pass on the virus and not be infected.

Some Airlines intend to fast test ALL before allowed to Board.

https://ca.travelpulse.com/news/airlines/westjet-a...

Not enough resources. Heathrow airport is working on being able to test all of their staff, but tests are expensive to produce and there are disputes over which are the most effective and unintrustive. 

Because airlines don't set requirements for countries to allow people in. 

Most places do not have the capability to do all those extra tests.  Testing tends to be barely keeping up testing people who might have COVID.  And results aren't instant, they take time.  So no extra resources to test those that seem perfectly healthy, but just because you want them arbitrarily tested.

And none of your predications would result from mandatory tests.  What if you take the test, then immediately catch COVID from someone coming into the testing area as you leave?  You now have a clean bill of health, but will be passing on COVID to a full plane of people.

That is being done in some cases.  However, it requires that the rapid result test be available.