This is one of a few areas where I think Marx oversimplifies things - mainly because he restricts the definition of “class” to an individual or group’s relationship to the means of capitalist production, and consequent self-identification and collective affinities as a consequence of that specific relationship. Because of this narrow focus, Marx then centered his ideas about class conflict around the bourgeoisie (those who control production, and benefited most from it) and the proletariat (those who don’t control production, and are exploited by it). And I think this was an overly reductive error.
As to why…well let’s start with some factors - in addition to, or aside from, control of production - that contribute to power differentials, freedoms, agency and so forth in civil society:
1. Economic status and mobility - from abject poverty to rentier, there are plenty of conditions and privileges that have nothing to do with control over production.
2. Race/ethnicity - this has a tremendous impact on freedoms, agency, opportunity, institutional bias, justice, etc. and also have nothing to do with control over production.
3. Gender & sexual orientation - ibid.
4. Native intelligence and levels of education or language ability - ibid.
5. Physical disability - ibid.
6. **Religious beliefs** - ibid (though more so in some societies or periods of history than in others…)
There are other variables, but this provides a general idea about how different “classes” of people can percolate up out of any given population, and how these class variables can potentially overlap or countervail each other. From thirty-thousand feet, Marx may have wanted to sort all of these different characteristics into his two major class distinctions, but that can result in a pretty inaccurate snapshot.
Let’s examine just one potent example to illustrate this point. A rentier does not - unless they are an activist investor - exert much control over production…if any at all. They are often purely beneficiaries of abstracted instruments of investment, having very little idea or concern about how their investments accrue, or how they impact society. So how, according to Marx’s definition of class, are they participating in class struggle? Through indulgent consumption of certain goods and services? Through supporting certain political causes? Through supporting certain types of capitalist? Okay…but what does that have to do with control over the means of production…?
Now what I do think Marx got right was that human history is very often energized by the struggle between the oppressors and the oppressed - within a given society, or between different societies. But this oppressive relationship can exist outside of the confines of control over the means of production (or exploitation by the means of production): to wit, women’s rights, or the cultural scapegoating of certain ethnic minorities, or prejudices around someone’s age or physical appearance, and so on. So while economic status certainly has a huge impact on oppressive relationships, so does the color of one’s skin (i.e. “white privilege”), or one’s gender, or whom one falls in love with, etc. Thus “class conflict” is IMO trumped by “the struggle between oppressors and the oppressed;” they may intersect, but they are not always the same.
My 2 cents.
From Quora: https://www.quora.com/%E2%80%9CThe-history-of-all-hitherto-existing-society-is-the-history-of-class-struggles-%E2%80%9D-To-what-extent-do-you-agree-with-this-Karl-Marx-quote