About 20 states have minimums matching the federal minimum wage. The rest are higher. Commonly the argument for a low minimum wage is because the cost of living is lower in some states. Also new entrants (teenagers) to the workforce would not be worth it to hire if wages were higher. Not hiring teenagers encourages them to do stuff that teenagers find to do when not working. Literally every dollar more it costs to hire manual labor leads to more job replacements due to automation. Automation is really the enemy of higher wages for unskilled workers.
Take for example Pantera sandwich shops. Instead of being greeted by a person your are greeted by a computer screen. They have invested in computers and a network to link the ordering system and POS (point of sale) to collect the payments. 2–3 jobs per store have been eliminated, a person or two to take the orders and 1/2 person to collect the receipts and count the money. This is a common solution to the cost of unskilled labor.
Congress could raise the Federal wage anytime but it’s not currently a priority for them.