To fully understand the scope of the television and film industries in their current formats, you should take a minute or two to browse television providers' guides, the Netflix dashboard, Hulu's selections, Amazon, and HBO, for example. Note the sheer quantity and impressive number of original programs and films available for enjoyment.
Remember that these shows and movies are the ones that made it. These works are the very best of the best that screenwriters put into the hands of executives, in the form of a script or treatment. To expand upon this point, a very small percentage of scripts are good enough to make it to the big or small screen. For every work that's on TV, hundreds upon hundreds-and sometimes even thousands- have been denied and have not been made to television or film.
The biggest reason scripts are denied? Lack of originality.
Frankly, by the time a script is reviewed by an agent or an executive, it's probably been established as "good" or maybe even "great." But if this script is unoriginal, there's a good chance it'll be tossed to the back of the pack.
Even when unoriginal television shows seem like they will be commissioned and are filmed as pilots, their lack of originality usually comes to light and they're shelved. For proof, just look at how many pilots are not submitted into series. The same thing happens in the film industry, albeit to a smaller scale.
At the same time, you can write a familiar advance in an original way. When done well, you may actually increase the odds of selling your screenplay because production companies and film agencies do not like to take risks and prefer to produce something that's been proven. It's a fine line that requires experience and feedback.