If you go into a romantic relationship, it costs you two friends. Those who have romantic relationships, instead of having the typical five 'core set' of relationships only have four. And of those, one is the new person who's come into their life.It takes substantial time to establish and maintain a close relationship of any sort. My recollection is that it takes 18 to 20 hours per week to be deeply intimate, like you see with lovers. The above figures suggest that a strong friendship takes around half that much time to establish and maintain.
There are only 24 hours in the day. Most people need at least ten hours a day just for basic life maintenance -- such as sleeping, eating and hygiene -- and most people need to work for pay, which typically accounts for another forty or more hours per week (not counting commute times).
Some people can work two jobs and stay healthy and maintain their sanity, but usually by sacrificing having anything most people would call a life. Others manage to work a full-time paid job while going to school full time, though usually for only a limited period while in thier energetic youth and usually at the cost of their social life.
If you look at both sets of numbers above, it fits with what we know of historical human relationship patterns in a very practical "physics-y" sort of way.
Most human cultures value committed, long-term, monogamous relationships. This makes sense on the face of it for reasons of germ control, purposes of providing for and raising children and other fairly obvious and frequently cited reasons why monogamy is best, which is often presented not as a best pracice but more like a moral edict from on high.
But it also makes sense within the above framework. You can have one lover, three good friends and a full-time job and this provides you a healthy, balanced life in various important metrics.
At least historically, some cultures were fine with a man having both a wife and a mistress, so long as he adequately provided for both and was good to both. This fits in the above framework in that logically it still leaves time for a full-time job and one good friend. It's also reasonably tolerable in terms of issues like germ control (assuming his two lovers are faithful to him), providing for the kids, etc.
Once you go beyond two lovers, it stops making sense. The numbers don't add up. You can't have a truly intimate relationship with three or more lovers and also work a full-time job and have any friends. There simply isn't enough time in the day/week to sustain that.
You sometimes see headlines about some American guy who secretly had three or four wives. If you actually read the article, he temporarily pulled this off by having a job that involved travel and having a different wife in different cities.
You also learn he was deceiving them -- they didn't know they were one of several wives -- and so these were not truly intimate relationships. You have to be honest with someone to have genuine intimacy. It isn't something you can have based on a pack of lies.
In the end, it winds up being a short-term arrangement that typically ends with all the women hating him and law enforcement of some sort wanting to speak with him (because polygamy is not legal in the US).
As stated previously, I think polyamory is real but real polyamory is rare. It's certainly possible to be genuinely intimate with two lovers. Lots of people have done exactly that in human history, if only by having both a wife and mistress.
But the idea that you can really, truly love LOTS of people is pretty much a fiction. You can have an open relationship and you can be honest with a lot of people that "You are not the only one." but most of those relationships will lack the depth necessary to meaningfully apply the word love to them.
Footnote
This post does not cover all use cases and is not intended to do so. There are cultures that allow for multiple spouses -- typically allowing for a man to have multiple wives but not for a woman to have multiple husbands -- and I assume that if you have three or four spouses and all live together, it's possible to spend a lot of time with all of them and know all of them reasonably well.But I don't really know enough about such to really cover that here. "So don't @ me" as they say on Twitter.