The concise answer is simple: Wealth begets wealth.
The reason that the richest top 1% of the world holds more than 50% of the total wealth is because the more money you acquire, the easier it is to acquire more. Despite the myths of meritocracy and endless opportunity, poorer people have little chance at climbing the class ladder because they have no resources and are infinitely bogged down due to difficulties that the wealthy never need to worry about.
Not to mention that wealth is passed on in family. Social class mobility upwardly is rare because of a common legacy in lower social classes of poverty. If a family never had one person to go to college, when one does indeed make it to college and earns more money, they will have plenty of relatives asking for help, not to mention that every step of the way will be extremely challenging, from taking out loans to simply not having social capital to figure out what to do next. In contrast, a higher social class individual is already imbued with plenty of social capital by their family, knows exactly what to do, will receive family loans interest-free, will not need to worry about much, and probably has a good idea of what he can do and what is at his disposal.
And besides, the capitalistic wealth system is imbalanced to begin with. The moment someone became incredibly wealthy was the moment a little bit of wealth was stripped from the rest of everyone else. The unrestricted amassing of wealth by few people results in little money to circulate and enable the enrichment of others.