Two things as introductions. First, nationalism is new, especially the idea that we should have states based on ethno linguistic religious identities. Maybe this started in France, Spain, and England during the early Modern Period, but the traditional date that most people give is the French Revolution, 1789. And from there, it spread to Eastern Europe several decades later, and really only started becoming important in Ottoman Anatolia, Levant, and Mesopotamia lands a little less than a century later. Second, people were and are mixed up. Any Sunni, Shi'a, Christian, Yezidi, Druze, Jewish, Kurdish, states would involve moving a lot people who lived in the "wrong places", according to the cartographers. And a lot of them wouldn't want to move. The Christians, in particularly, would be a sticky minority because there are a lot of them, but they're spread pretty thinly and splotchily throughout the entire region.

So, yeah, it's important to remember that nationalism was relatively new in the eastern part of the Ottoman empire at this time. Also, that earlier nationalism was largely linguistically based. Germany had just united and was about two thirds Protestant and one third Catholic. There was some political wrangling to ensure a Protestant majority by keeping Austria out (the so called Kleindeutschland solution), but no one thought that this made Germany inherently unstable, though it did lead to Bismark's Kulturkampf which sought to reduce the influence of the Roman Catholic Church Catholic majority Bavaria. Similarly, the South Slavs were uniting for the first time in an independent state, and no one thought that the Catholic Croats and the Orthodox Serbs was a bad idea. Obviously, people thought at the time, like the Germans, they were one folk whose power had been divided unfairly into multiple states.

And that's actually exactly the critique raised around the French and British mandates: it's not that they were too united, but that they were too divided. This was a criticism of the French in particular. Lebanon was designed as a safe haven for Arab Christians, an artificial little slice of mountains and coasts where they were supposed to have a demographic majority perpetually (birth rates disrupted this). The French also originally carved out states for the Alawis and the Druze, and made the Sunni Arabs into two administrative region, on centered on Damascus and one Aleppo, but protest disrupted this. Of these, only Lebanon remained independent because, well, Christians had a lot more pull with the colonial powers than the Druze or Alawis. After breaking off a piece for the traditional Maronite stronghold, they basically left the largest state possible. Britain did a similar thing with Iraq, though they broke off Transjordan because of considerations for Zionists, Holy Jersalem/Bethlehem, and proximity to the Suez/Bethlehem (I think those were the reasons, that's the thing in the post I'm least confident on). But the main issues, especially among the effendiyya middle class, in the Transjordan and Iraq weren't "We all hate each other" but "We hate that we're not in one state together." I know less about Syrian policy because, well, French, but I assume it was similar for the most part. Pan Arabism was the nationalism that really mattered at the time for most of these states, not sectarianism and not the state nationalisms (Palestinian, Egyptian, etc). There was considerations for a Kurdish state on the Turkish side of the border which was likely going to be extended to the Iraqi side of the border was considered, but Turkish military victories in the period directly after WWI dropped this idea, though I don't know any details of British Kurdish policy in the mandate era in Iraq.

So, no, the main question was why so many borders, not why so few. And even if the French and the British had wanted to divide people up, that would have been impossible. The only reason borders are so neat in Western Europe is because the French made all the Occitan people speak Parisian French, and the Castillians tried to make the Catalans do the same with Catalonian. But even here, there are exceptions: look at Belgium (sorry my dear /u/estherke), what a weird state... no self respecting nineteenth century nationalist would ever design that. In Eastern Europe, this was only made possible with massive expulsions and flight so that people "unmixed". In Turkey, and Greece, this happened before and after WWI. For Poles, Jews, Czechs, Slovaks, Germans, Ukrainians, and Albanians in Greece, this happened mostly after WWII. Hungarians were never resorted, nor were the Turks in Bulgaria and Macedonia, nor were Albanians in Yugoslavia. This was the more typical settlement pattern for the world before the massive European ethnic cleansings of the first half of the twentieth century. But look at a preWWI ethnic map of Europe and notice all the little smudges and polka dots, and realize that this leaves out not only people like the Roma and the Jews, but also all the mixing where people were relatively small minorities. Now look at the Middle East, here's the French Territory of Sykes Picot. Now draw states where people aren't mixed. My buddy Nick, who runs a great blog called Afternoon Map, has several more in his piece in the Atlantic, 15 Maps that Don't Explain the Middle East.

But just keep in mind that people were very very mixed already; it's not like the French and British mixed up them up, that's just how people lived. Always, until nationalism. An Armenian village there, a few Arab villages there, a Circassian villages here, some Turks, some other Christians. Then in the city, it's even more mixed up. The alternative to leaving people mixed like they'd always been would be a massive ethnic cleansing, which besides, being ethnic cleansing, would be expensive for the French and the British. The two states besides of Europe around the World Wars who tried something like this in the Twentieth Century were Muslims, Sikhs, and Hindus at Partition, which left millions of people displaced for years, tens of thousands of women raped, and hundreds of thousands of people dead. It's one of the great tragedies of the last century. And any umixing of the Middle East would surely have left us with something similarly tragic.