The same question has been asked at the places I work…
The best answers we can come up with are;
Some filmmakers won’t acknowledge how many people watch their movies on other devices.
Some filmmakers only regard cinema as the real thing, everything else is a side project at best (one they might work hard on, but it’s not their primary concern).
A lot of filmmakers do it because everyone else is doing it.
And as noted in the article posted recently; some people think it sells more tickets.