Below a certain income, an average lower-middle class family will generally get more back in tax refunds than they pay in income taxes. A family of 4 making 50k per year (fairly livable in much of the country actually!) might only pay in 3-4k in income tax but then receive 6-8k in credits for the child tax credits, earned income credits, etc.

And for a lower-middle class family that tends to be the biggest amount of money they see at any given time. They're only bringing in about $2500/month after taxes, retirement contributions and insurance, and that's got to cover all living expenses. Recieving a nice big check each year tends to be the best way to be able to make a comparably large financial move, often paying down debt that's built up over the year, maybe buying a new car (I know I did that one year, put the entire tax credit into a used car, and that was the very first car we owned outright!) or if it's been a good year that money can be spent on fun stuff

The other side of the coin is that for the overwhelming majority of people who's earnings are coming from their jobs, the amount of money that gets withheld for taxes simply isn't enough to make enough money in interest to be worth while. If you pay $300/month in income taxes (not unrealistic) it would take 4 months before you've got enough to buy the smallest short term CD you can, then that 9 month CD for example will only earn you about $30 before you cash it in in February to pay your taxes. Basically in order for placing your tax money into safe investments to pay enough to be worthwhile, your tax bill would need to be so large that you'd already be paying quarterly anyways and already have much bigger fish to fry than worrying about your tax withholdings not earning you interest.