Because the Americans, like 99% of other countries, are only in favour of free trade in areas where they have the economic advantage. Protectionism is sometimes necessary hence why the UK is currently piling on environmental and agricultural legislation post-Brexit; nobody wants to become a dumping ground for heavily subsidised US agriculture exports (which is the real reason why the EU bans chlorinated chicken and so on, citing 'safety issues' is just a convenient excuse).
Tariffs are old news anyway, it's far easier to simply regulate foreign competition out of your domestic market these days. The EU is quite good at doing that and then exporting said regulation overseas-essentially countries have a choice to conform to either US or EU standards/regulations. They're the two biggest economies on Earth and have competing regulatory systems (e.g. if you've ever bought American chocolate in an EU supermarket you'll notice they have those weird sticky labels on the back to meet EU standards). That's the real damage that Brexit will do to the EU-if its economy shrinks by c16% then aligning to US regulations becomes more attractive as it has a far bigger market share.
