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I am wondering why there are so much airplanes made in USA in the world's air forces prior to WW2, since I don't think the US Air Force had the better reputation.
You can count the:

• Glenn Martin fighter


• Brewster F2A Buffalo fighter


• Martin Twin engine bombers in multiple variants


• ...

How did the US, with their isolationism, concluded so many contracts? And why only in air forces and not in naval or land warfare?

I am not speaking of the lend lease hardware.

US actually didn't sell so many planes between wars

Although US was industrial powerhouse in that period (something like China today), military planes were still something something new and experimental for most smaller and poorer countries. Yes, they did participate in WW1, but question remained how effective they were and how effective they would be in future wars. How much should country spend on military aviation, should they import them (and risk dependency and obsolesce in fast changing field ) or should they try to develop their own.

Most medium-sized countries tried to thinker with their own domestic aviation industry, at the same time importing limited quantities of foreign built aircraft. Initially, US companies were not particularly successful, because US aircraft industry was not especially advanced. For reference, in WW1 US used almost exclusively French built planes. In fact, between wars, French aviation industry was dominant, and sold lot of planes like for example ubiquitous Breguet 19.

Situation changed only in late-thirties, as war became more and more certain. Reasons for this were two-fold. First, technologically US became one of few countries that could independently produce modern aircraft engines. Engines, then as today, separate amateurs from professionals in aviation business. Countries that could develop engines are free from restrictions what kind and how many aircraft they could produce . Second reason was US relative isolation and neutrality at the beginning of the war. While other industrial nations prepared themselves for war, therefore having limited capacity for export, US was able to use its geographical position to actually sell military planes to them. One example would be French desperate purchase of P-36 Hawk. Although French arguably had better designs in pipeline, sheer need to increase number of available planes quickly forced them to employ US aviation industry.

As for your question about land and naval forces : before and at the beginning of WW2 Allied countries (France and Britain) had clear naval superiority over Germany, and even over Germany + Italy. There was no urgent need to buy foreign ships with their own shipbuilding industry being vastly superior. Situation did change when Germany started to use large number of coordinated U-boats (Wolfpacks) and Japan joined the game. After that US shipbuilding industry got into full swing.

As for tanks, artillery and other land based equipment, US Army was a bit of orphan both then and now. US policy of isolationism emphasized strong Navy and latter strong airforce . US Army on the other hand still used lot of WW1 equipment. First US built tanks appeared only in 1934-1935. Only after lightning German victories at the beginning of the war US reshaped its stance on problem of armored warfare.