Money.

TV is like making cheap movies. As an investor you can calculate your return on investment every time an episode goes to air. If you’re on a winning streak, you’ll make more, if not, you’ll pull out and the show is cancelled. Many TV shows never get past the first pilot episode.

Making movies gives you potentially a bigger payout, but only if it’s a hit, so an investor will do everything to maximise the chance of success. Massive budgets, known stars, etc.

In the end it’s all legalised gambling. If you’re lucky, something interesting comes from it, more often than not it’s Waterworld … yes, there are worse movies, but you won’t recognise them … and yes, it made a profit … eventually.

Worst performing movies of all time: https://en.m.wikipedia.org/wiki/List_of_biggest_box-office_bombs