Why did scientists pre-lavoisier believe that matter was destroyed in the burning fire experiment?

Before Lavoisier, the father of Chemistry, scientists thought that every combustible substance lost matter when burned because they observed that a residue of less weight was left after combustion.

They reasoned that this loss of matter was due to the loss of a substance they called phlogiston to the air when the substance burned to make fire. 

Lavoisier proposed a theory in 1777 that excluded phlogiston and explained that combustion was the reaction between a metal or an organic substance with a part of air he later called oxygéne, from the Greek words for "acid generator". He could propose that matter was neither created nor destroyed in chemical reactions but transformed. 

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They did not study the continental drift