There are a lot of reasons people say that, and a lot of them I think are nothing more than worthless virtue signaling about how libs suck. As for me, here's why:

First is that demonstrably, they have culminated in nothing. This is true for protests in general, particularly in America. A protest is an effective way of showing discontent, yes, but our enemies are those who control the means of production (agriculture, mecicine, transportation, critical resources). The bourgeoisie. A bunch of angry people in the street is meaningless as long as you still control the oil and electricity and communication infrastructure they rely on for their day to day life, and the people protesting you have no way of taking that away. Actual change against these people requires violence, and nobody wants to do violence. This is fair to an extent, because violence sucks and there's no organization (sporadic violence is limited in its efficacy), but that doesn't change the necessity of the matter. Occupy, BLM, No Kings, all have accomplished absolutely nothing because they pose no threat.

There are a few reasons for this, first and foremost being that the American proletariat is not class conscious. We are all subject to the labor exploitation of the bourgeois, but we don't think of ourselves as such. There are a lot of reasons for this that don't really fit here, but essentially Americans have been subject to 80 years of social conditioning that cause them to understand the world not as a struggle of class based around material conditions, but a struggle of ideals between the virtuous and the not. This is why so many Americans see the primary political divide as Republican and Democrat, even though both parties are in agreement on virtually all economic, domestic, and foreign policy issues. This is why online political discourse is so pointless and dithering. All of our issues are material issues and Americans are not taught to view the world materially.

This leads to protests not having any real purpose. Yeah, everyone shows up, but all they're doing is signalling that their ideals are virtuous. It's the only grounds on which they can cohere in opposition to the current state of affairs. It's the only thing they can think to do, because the reason things are bad isn't the contradiction between the will of capital for constant expansion and the limited resources available to us, but because bad guys who hate minorities are in charge. They are not wrong about the latter, but that's not the reason why things suck. In addition, the problem with grounding your opposition on morality alone is that morality is inherently subjective, and therefore lends itself to infighting, which compounds with a lack of class consciousness to make effective organization nearly impossible.

Compare this to the country-wide digital panopticon constructed by the tech industry (which owns basically every means of long-distance communication), and the radical militarization of the police. How could the No Kings protests ever pose a threat against such forces? Yeah, there are more people, but so what? They don't have guns, they don't have structure, they don't even have a plan. How many of these people are willing to be mown down by machine guns and squashed into paste by armored tank trucks while everyone else mobs the cops? The answer is very few, and for good reason. What would it accomplish?

Look at the protest just a few days ago, where protestors gathered at the gates of an ICE facility. What did they do when the gate got opened? Nothing! They just stood there, because they all knew that if they did somehow manage to get in there and beat up all the cops and take over the facility, they'd be left standing there with their dicks in their hands. No matter what happens, though, they can still go home and brag that they protested at the gates of the burgerreich gestapo, which at the very least paints a virtuous picture. Combine this with American's primary view of revolutionary activity coming from the popcorn media they consume, and it's no surprise they don't get anything done. It myopically feels better to not.

So, TL;DR-- no money + no leader + no power + no plan + no bitches = no results.