Mostly because it is a different system. The parlement in France doesn't elect the government. The president choose the prime minister, and the prime minister form the government.
The parlement can, at basically any point, vote to "censor" the government, effectively terminating it if a majority is reached (they do stay in position until a new government is formed, but can only take care of the "affaire courante", i.e simple daily affairs, no big laws, reforms, policy changes, ...).
Indirectly, this means that the parlement can have a lot of power over who will be in the government. If one party has an absolute, or almost absolute, majority in the parlement, it more or less mean that they can decide who shall be in the government.
Due to the way our election works, the party of the president usually has the majority in the parlement. This is even more true since Chirac changed the duration of the presidential mandate to match the duration of the parlement mandate (and their elections are very close). This usually give a lot of power to the president, effectively making him the sole judge of who shall be in the government (since he choose the prime minister and the parlement member of his party follow his lead).
But what we have right now is a very unique situation. Nobody has even close to an absolute majority in the parlement. The left "union" (which is not really united, but that's another story) control roughly ~33% of the seat, the extreme right party control ~21% of the seats, the presidential party control ~15%, and the rest are various centrist and right wing party.
The issue is that there is no way to get a solid absolute majority with this assembly. For the left, even if they were to compromise heavily and add as much center-right as possible in their coalition, it would barely give them a majority, and the government would have to compromise so much, it would piss off all their voters, which would be a political suicide for the next presidential election. Nobody want to ally with the extreme right except a few members of some right party, which would never give a majority (and the RN would only go to the government if they don't have to make any important compromise, for the same reason). So, you have to remove the two biggest block. And so, even if all the other party would somehow make a coalition (which is more or less the government we had since the assembly dissolution), it would still give less than ~45% of seats in the parlement, not enough to ensure that your governement would not be censored (which is what is happening, we've had like 3 or 5 government since the dissolution, i can't even remember all of them).
This is an unprecedented situation in France and basically the achilles heel of the fifth republic. Our system cannot function without a clear majority in the national assembly, and the way the assembly is divided right now makes any majority coalition impossible (or a political suicide).