I don't really think there's a need to overfly AMS/CDG. By concentrating all the SEA originating traffic into the two hubs, they can support two flights per hub per day - if it was peanut buttered across other destinations, the viability of AMS/CDG would be affected. And the timing for the hub routes tends to be really convenient, other than the early morning departures on the return from Europe, so it's not like the existing schedule is driving people to the competition.
Also, SJC ORD LDR is right that SEA doesn't make sense for connections to Europe. From cities like PHX or DEN, Seattle tends to save mileage over any other connection point to Asia - SFO, LAX, SLC included. But to Europe, the situation is reversed - Seattle would be at least 500 miles out of the way, if not more. Not a competitive option, when that traffic could flow over SLC and support expanded international service there.
Finally, SEA is heavily constrained by the international arrivals facility right now, and Delta wants to save all the capacity they can for Asia flights - there simply isn't customs capacity to handle random flights to Europe.
In the specific case of Italy, I do think a SEA-FCO/MXP flight on Alitalia could make a lot of sense someday, if and only if they are able to get their financial house in order. That would also serve to help SkyTeam capture yet another new-airline-entrant incentive from the Port of Seattle, as an added bonus. But that flight, like AMS and CDG, would be propped up by additional connecting opportunities.