You're forgetting about the people who are only working one part-time job too, like students, the elderly, or 2nd-wage earners in the household.
There was a time in the very recent past that single wage earner households were the norm. Cultural changes have increased two-wage households but the stagnation of wages has also played a large role, and there are plenty of people disinterested in the status quo.
UBI doesn't have to be higher than the "average" income of a low wage worker, it just has to be high enough to offset the marginal utility of working for some people.
Working isn't full net benefits, you have to consider taxes, the costs of commuting, typically elevated meal costs, costs related to equipment/clothing required for that particular job, and especially childcare costs. Then there are opportunity costs, the value you place on your own time, and people who just plain hate their fukking job or their boss. There are a lot of people who would take a cut in pay if it meant they got to devote that much more time to church ministry, environmental activism, art, their musical/artistic endeavors, etc. Or who would finally find enough margin to go back to school.
You're also neglecting that in a multiple-adult household, you'd have two UBI to replace the missing salary, not one. If UBI was $1000 per month per adult, then a two-parent household who had taken in one of their elderly parents would now have $36000 extra to work with. That would easily replace an entire salary in many households, especially when you consider that the extra savings on childcare/commuting/food could be another $15000 or more.