Basically, as a fellow Brit you will know that for most people your tax just happens automatically (typically through the PAYE system, so tax is taken before you get your paycheck, and we never have to even think about it).

Then in the UK if you have more complex tax affairs, then you have to fill in a self assessment on the government portal (where it already knows most of your tax information, and you are pretty much just making disclosures).

And it's VERY rare that people have to hire an accountant (only for people who own businesses, or individuals with the moxt complex tax affairs).

Well in the USA they don't have any of that, and everyone has to file their own taxes. The USA has the capability to do automated tax like in the UK, however the companies who sell the tax software petitioned the government to make sure that they didn't implement it so they can continue to get $$$ out of everyone else.