My understanding is the milder climates in Europe are caused by there being an ocean to the west of it - the western United States gets the same sort of temperatures.

Very little of the western US has temperatures as mild as are typical in Europe. Once you go even a few miles inland, extreme temperatures become common. For example, the Inland Empire suburbs of LA are notoriously hot in summer, and even temperatures in the 100s (Fahrenheit) are normal during summer.

To be fair, Europe has vastly more coastline, and directly along the west coast of the US weather is extremely mild. LA is notoriously hot but it also doesn't get cold there; the temperature range for any city along the west coast is probably 50 degrees from cold winter days to hot summer days [citation needed].

Also, Firenze, Italy has some pretty extreme weather and is only 90 km (55 miles) from the coast.