Pilot salaries and fuel costs both skyrocketed since 2020, and at the same time the US got significantly richer and people got more of a taste for luxury. Expensive, premium things have never been doing so well in the US. All of a sudden, you have a double whammy of low-cost carriers being forced to raise prices and get less competitive, while consumers are more inclined to pay for luxury.
Not to mention - American LCCs were never in that great of a position, structurally, vs the European LCCs. Ryanair thrives on quick turnarounds, cheap airports, cheap pilots, short flights and high population density. The US has none of those. The EU's population density is 112 people/km2 vs 36 in the US, even cheap US pilots are very expensive, turnarounds are slower, and there are few budget airports where the people live. The Port Authority is spending 25 billion dollars on revamping the three NYC area airports. Meanwhile Europe is full of cheap semirural airports like Frankfurt-Hahn, where the most recent renovation cost in the single digit millions. Where do you think you can land a 737 for cheaper?