There are plenty of longer bridges, but those are multi-span bridges. The Straits of Messina are extremely deep and the area is tectonicly active. So it isn't feasible to build supports in the middle of the strait. If it were feasible then it would have been done a long time ago.
The design calls for the longest single span suspension bridge in the world. By a lot. It also needs to hold up to powerful storms.
The other alternative would be a pontoon bridge. The one in Seattle crossing lake Washington is 2km long, so that would work (https://en.wikipedia.org/wiki/Lacey_V._Murrow_Memorial_Bridg...). However, The Straits of Messina are one of the most active shipping lanes and the maritime traffic is more valuable than the road traffic. The pontoon bridge works in Seattle because the main port is in the Puget Sound and there isn't any major shipping in Lake Washington.
> The pontoon bridge works in Seattle because the main port is in the Puget Sound and there isn't any major shipping in Lake Washington.
The old 520 bridge (longer than the I-90 bridge) used to have a drawbridge in the middle because there was a naval station in Sandpoint. The new 520 bridge doesn't have it, and the naval station has long since been closed down. The hood canal floating bridge gets a lot more traffic although mostly Submarines I think.
Not to mention the fact that the economy of the whole of Sicily amounts to about 90 billion euro, the neighboring Calabria amounts to 35 billion and the nearest major city to Sicily is Naples, 480 km away.
Shocked that they don't have a bridge already.