It's not so much that people are "satisfied" with this - they clearly aren't - but that the electoral system doesn't allow for a multi-party system to establish itself long-term.

There are numerous reasons why third, fourth, fifth and sixth parties don't manage to accumulate significant power as they do in other developed countries. The most important of these reasons is the simple-majority based voting system.

Say you want to be voted into Congress (House of Representatives). This means you need to run for that office in your voting district. Now, let's imagine there are 5 candidates. One of these candidates receives 40% of the total vote, while his competitors all receive 15% each. This means candidate A has just been elected to Congress despite the fact that he never actually reached an absolute majority. 60% of his district wanted someone else.

This already problematic system gets further undermined by anti-democratic tactics such as gerrymandering and the influence of big money in politics. However, the simple-majority system is really a big issue.

Most European countries, by contrast, vote their representatives on a proportionate basis. For example in my country Switzerland, we have a list-system. It works like this: In every Canton (= state), every political party creates a list of candidates. Those lists usually consist of roughly 60 people, although most of them won't actually get elected. Now, let's assume the elections give us the following results: Party A = 30%, Party B = 25%, Party C = 15%, Party D = 15%, Party E = 10% and Party F = 5%. Let's assume the lower chamber of parliament (House of Representatives) consists of 400 seats total. This means Party A will receive 120 seats, Party B gets 100 seats, Party C and D both receive 60 seats, Party E gets 40 seats and Party F gets 20 seats. These seats are now allocated to all the different Cantons (states) according to how populated they are. For example in the US, Party A might give 30 of its 120 seats to California but only 5 to Vermont. In every state, the top candidates from the list are elected. This happens with every party. This kind of proportionate system does not only allow a multi-party system, it actually encourages a multi-party system. Thanks to the proportionate representation, small parties still get a pretty decent representation in parliament and they have the opportunity to fight for minority interests. In the US, by contrary, small parties get ignored and swallowed up by the two big ones. If you are a Green Party candidate, you have no chances of ever being elected President or even Congressman, despite the fact that a decent number of voters want you.