The issue with your suggestion is that it dis-incentivises work which is one of the problems UBI tries to solve about welfare systems. Currently the issue is "why would I work more than X amount if that means that I make less than if I worked less than that (welfare + work)?" but with your system it becomes "If I am relatively unskilled, why would I try to work at all if that means that I will receive almost no compensation beyond what the government already gives me?".

UBI is supposed to support people so that they can do the work that they believe is most valuable and to prevent people from being dis-incentivised from pursuing work. It has other goals as well such as eliminating other welfare services, removing minimum wage, and incentivising moving away from cities but those first two are the primary reasons.