At a fundamental level, class struggle and the theory of the state means the working class must overwhelm the capitalist class, and this cannot be done within the framework of existing, bourgeois society. That's why all lasting socialist states have come through revolution.

The problem is that a capitalist system will not allow itself to be reformed in this way, as the "reforms" that Marx poses are antithetical to the very foundation of capitalism.

To give some accessible examples; you can't house homeless people or give people healthcare and higher education because homelessness and debt is a whip to keep the workers working for whatever wage and conditions are offered by a capital owner. You can't deconstruct racism because it was invented in the first place to keep the working class at war with itself rather than struggling against the conditions set by the ruling class. You can't stop imperialism because infinite growth requires infinite and unrestricted expansion into new territories.

The system of capitalism manufactures its own required conditions through cruelty and social inequality (and yet, it's these very things that lead to resistance), and without those necessary components the whole system collapses. The ruling class will not allow this to happen, because this system serves their material interests, and thus fundamental change cannot happen until the working class; whose material interests are directly opposed to those of the ruling class; is in power. The ruling class will pay lip service and the occasional half-measure in order to obscure this reality and make "reformism" seem possible, but 1) that is all they will do especially in the absence of a real threat to their power and 2) they will always eventually claw back even the smallest and hardest-fought of crumbs. Crumbs are good and all but there comes a point where our energy is better spent fighting for the whole cake.