The reasons that Americans drop out of college are primarily financial. The cost of attending college in the USA is far higher than in European countries that have A levels, and students who dropped out put financial reasons as #1. Only 10% of the students who drop out say that classes being too difficult was their major reason. Another 24% of all dropouts said that difficulty of classes was a minor issue. On the other hand, 54% of the students who dropped out said that the need to make money was their major reason.
Also, the graduation rate from the most competitive colleges in the USA is identical to those of the most competitive universities in countries that have A levels.
The difference is that UK students pay less and have governmental loans that are close to the tuition rates, meaning no need for private lenders.