Probably in part because hanging on to wealth involves some degree of stewardship. Inheriting wealth is the best way to get rich, but you can inherit wealth, run through your inheritance and end up a pauper in short order.
Of course, that's abusable because "he who has the gold makes the rules." People from wealthy families tend to be in a better position than poor people to influence the laws that get written and this tends to foster a situation where "them that has, gets."
This is something we need to guard against. In order to have a healthy society, we need to actively work at making sure that being born into the lower classes (or falling into them) isn't a trap because of wealthy people actively sabotaging the efforts of poor people to build something of value and accrue value to themselves legitimately.
I'm not for UBI because it breaks that connection between wealth and the creation of value. It actively encourages people to forget how to be productive in a way that is plugged into the larger system (I'm a former homemaker -- I assure you, knowing how to be productive in a way that is good for your own life doesn't automatically teach you how to be productive in a way that plugs you into the world of paid work).
In order for human society to continue to have wealth, there must be incentives for all humans to find ways to add value to the system. One of the ways to add value to the system is with good stewardship.
It may not seem obviously like a skill, but I spent some time reading every article and watching every TV show I could about what happens to lottery winners and two-thirds are bankrupt within five years. So simply being handed a whole lot of money doesn't, per se, tell you how to keep it.
The concept of UBI is a concept of wealth as a giant pile of gold somewhere that we can just tap into. It's a broken mental model.
Societal wealth is an abundance of goods and services and it takes human wisdom and human effort and human self restraint to make sure societal wealth lives on and doesn't disappear suddenly. Once it's gone, it's a lot harder to recreate than it is to maintain the existence of.
I think UBI is a recipe for killing the goose that's laying the golden eggs. Our challenge currently in the world is to figure out where to go from here without doing that.
Stewardship (without being an abusive classist asshole crapping all over the lower classes and treating them as your de facto slaves) is a valid part of finding those solutions so we can foster a golden age instead of stumbling into a dark age as our next stage.