The casting couch has frequently been used as a joke over the years, but it clearly came from a place of people's real experiences.
In the early days of the big studios, during a time when women had no power and no say, abuse and harassment happened as if it were a part of the movie-making process. This practice may have decreased some over the decades, but it must have still been a somewhat common occurrence.
To shorten my long response, there have to be many people who are still uncomfortable talking about their experiences for all the reasons it's normally difficult talking about them.
This is one aspect of this I go back and forth on. There have to be false allegations out there, but there is no way to know many. It could be 5% or 50% and it's not possible to figure out if a lower or higher percentage of the accuser are making it up (my tendency is to assume lower, but I can't know).
The other part of the false allegations aspect of this situation is that people use it to dismiss all allegations with statements like "There are false allegations, so probably none of it is true". That's not helpful to the discussion at all.