Huh? What do you mean?
It’s the basic probability curve.
The proclivity of people to engage with something takes effort, it takes more effort to stay engaged. The longer the engagement lasts the higher the chance for something else to interrupt it.
I could go further into the psychology of it and how attention and hobbies and so on interact with each other but that’s a bit too much for that.
Suffices to say that it’s obvious that less people finish something then start something first and foremost, a logical conclusion.
Also the chance for more and more people to not engage with something the longer it lasts is progressively higher over time until you only have ‘mental outliers’ (we all are in some aspects, that’s normal) leftover.
So the logical conclusion - and a proven one via statistics and aligning with psychological findings actually - is that less people then buy the game start it. Less then start it actually do more then trying it out, less then those trying it out finish the campaign, less then finishing campaign will finish base monolith, less then reaching empowered monolith will reach corruption 300 and less then reaching corruption 300 will reach corruption 1000.
This is a nigh infallible thing - and worth a case-study if broken - since each of those steps takes progressively more effort to achieve, hence leaving people reaching their ‘limit’ in terms of engagement behind.
Same as with any hobby, any goal, any career. It’s why only very specific people take up high management positions of big companies and they generally all - with very very few exceptions - have the same basic emotional indicators in the extreme. Little compassion, highly internal thought processes, highly goal-driven. Why? Since it’s a competitive environment at the highest level and otherwise you’ll fail to be in it, having to sacrifice a otherwise ‘normal’ social situation by default.
See? Then you understand what I’m talking about! 
Yes, and that’s why in comparison to PoE we can’t go with the same notions inside which is ‘viable’ or not. A PoE player can easily enjoy the game without being able to complete all the content. In Last Epoch? We don’t have as much content, not finishing some parts of it is a much bigger issue still. We have too little to do for too much time while PoE is too little time for too much to do.