Question: how do these people on UC who get paid monthly now think us working people get on?
Firstly, a fair sized proportion of people claiming UC are also in work. It is not solely an unemployment benefit so it's not a case of "us working people" vs "them". Check your facts.
Secondly, people migrating onto UC from other benefits (e.g., JSA) will have been accustomed to being paid weekly and don't have the funds to suddenly accommodate monthly payments. Imagine your employer pays you weekly for years and then all of a sudden announces that they're moving to monthly pay, you'll get paid this week and won't get paid again until one month from today. Could you survive that transition period without getting into debt or hardship if you had were already living hand to mouth?
Thirdly, some vulnerable people on benefits don't have the organisational or budgeting skills to deal with monthly payments. They have chaotic lifestyles or neurological disabilities or addictions or other difficulties. With weekly payments if you receive money on Monday and it's all gone by Wednesday, you only need to wait until Monday again for the next payment. You can ask people you know if they can lend you a tenner until Monday, you can stretch what food is in the cupboards for a few days more, you can go to bed early the rest of the week to save on light and heating. There are ways and means to get through a few days. When it's monthly though, it's harder. Can you lend me a tenner until the end of the month means the lender is without their tenner for a lot longer so is more reluctant to lend it out in the first place. You can't stretch three days of food into 28+ days of food. You can't go almost a full month with barely any light, heat, or hot water. Budgeting can be taught, but no one has taught them. The DWP didn't equip any of the unequipped people before they threw them in at the deep end and told them to get on with it.
And finally for people who are in work, UC is calculated monthly and if your wages happen to be paid twice within any given assessment period (e.g., if you are paid four weekly) then you get no UC at all that month even though you haven't had any extra salary. Imagine you're working full time on minimum wage, your wages cover the rent, the council tax, and your travel costs to/from work while UC pays for everything else (food, gas, electricity, water, childcare, clothing, etc). Where do you then find the deficit from?