Simple answer: because English requires the definite article. It's not optional, as it is in some languages. Thus, if you want to talk about the current heatwave, you simply cannot drop the article.
You could modify it a bit and say "this current heatwave": that could emphasize that you are talking about the current heatwave only, and not other heatwaves. The use of "the" doesn't really emphasize that the current heatwave is different from any other: in that sense, it's more generic, required by English grammar but not really making any particular emphasis. What one of the comments on the original question said is correct: you could change this to simply "during heatwaves", and you'd be saying basically the same thing. All "the current heatwave" implies is that (1) there's a heatwave going on right now and (2) people are ignorant of risks during heatwaves in general, the current one included.
However, don't expect perfect consistency by people. I could easily see a TV reporter, for example, saying "this current heatwave" and meaning nothing more than if they just said "the current heatwave" or even simply "heatwaves." They may be just emphasizing, yes there is really a heatwave going on right now, and by golly we're bringing you the most news of any news outlet about it. In other words, using the emphatic "this" to be more about their own great reporting work than about the heatwave itself.