But the money doesn't just vanish when it enters people's bank accounts. All of a sudden, banks have a lot more collateral against which to borrow and invest, by virtue of the fact that the bank accounts which store the UBI have money in them. People spend that money and it enters and moves through the economy. The government taxes it back out via sales taxes, income taxes, capital gains taxes, etc.
I think, in general, it's worth having an economy that keeps moving. If you let too few people have too much proportion of available wealth, the consumers won't have any to work with and your economy will be entirely dependent on luxury goods for a tiny proportion of the population.