Browser development costs money. However, if developers started to charge for downloads, consumers would shift to the other options which are for free. This is because there is so much market competition that browser developers cannot afford to charge money for them. Instead, they get revenue with shares and advertisements and the other features that are associated with the browser rather than directly from the consumer.
This is why browsers are for free. It is all a vastly linked network that ultimately leads to exchange of cash and earning by using customers one way or the other. You may not be directly paying for the browser but you are viewing an Ad that has paid the browser for advertisement.