When observing people shadowboxing, it seems like Karate is a more pretty, artistic, and fancy martial art. Muay Thai seems more brutal and ugly looking at some times (even though I practice it). Did Karate purposely try to make kicks more artistic/fancy or is something that happened unintentionally?
As ever so often, this is a result of the ways of training prevalent in the respective arts. Even if not intentional as such, it is a logical outcome of the history of (Shotokan) Karate:
Originally, (Okinawan) karate did (and to this day does) contain much more partnered exercises and full contact sparring in small training groups with intensive use of individual instruction (used to be classical in-house training). This changed when Gichin Funakoshi (himself a rather mediocre practitioner in Okinawa) introduced his "Shotokan Karate" in mainland Japan. It was seen as the perfect "Japanese" complement to Judo, containing mainly strikes and kicks - the main reason why Shotokan lacks all the grappling and close-combat application that is part of Okinawan karate in order not to "compete" with Judo there.
Funakoshi may not have been the best Karate student himself, but he certainly was a cunning political and economical mind. Thus, he attracted a lot of students and acquired help for the spreading of his new style all across mainland Japan. He introduced the belt system known from Judo. And as the groups were so huge, the training of technique, much as the grading, shifted from partnered exercises and conditioning to be almost entirely based on individual forms.
As a consequence, the techniques developed a more prominent aesthetic dimension as they embodied an "ideal" technique which, following kinaesthetic and physical principles, maximized the speed of and control over the individual technique, but at the same time increasingly developed it away from actual practicality and applicability in a fight, as it was never incorporated into a system tested in and fit for fighting. For example, the classical long, low Shotokan stance is good as strengthening, flexibility, and coordination exercise but completely impractical for fighting; other karate styles have higher, more natural stances. A lot of the alleged mythical principles of "karate" like the one-hit-victory through a single perfectly executed technique are actually an after-the-fact justification for this kind of training.
Up to this day, speed and control are what is valued the most regarding the grading of a technique, may it be in Kata, Kihon (repeated execution of individual technique), or even Kumite. In Kumite, people can be disqualified if they hit their opponent too hard as it is interpreted as a lack of control and restrain.
As a matter of fact, Funakoshi time and again was ridiculed for his forms of training by former fellow karate masters from Okinawa, pointing out that what they learn was not how to fight but how to look good by pretending to do so. He was even challenged and beaten in his own dojo, repeatedly, within seconds, to make exactly this point.
This did not stop the success of his Shotokan Karate, which is the most spread and influential form of Karate up to this day and, eg. in the form of Taekwondo, gave birth to other branches and arts (which inherited its problems and virtues).
Now, compare this to Muay Thai: Continuous stress testing of techniques which leads to the omission of once-in-a-lifetime moves and a reduction to fast, effective, non-telegraphing techniques. It is only natural that this looks very different and less aesthetically appealing since the beauty of the form does not bear any advantage at all.
Disclaimer: I do think that Shotokan karate has its virtues as it can optimize a lot of lacking technique which was never questioned as it "worked" well enough. At the same time, it has overdone doing so without re-incorporating them into full-contact stress tests, which means most practitioners cannot actually fight as they never did. Also, there used to be full-contact Kumite and there is a lot of drive towards re-incorporation of full-contact application in the Karate community both in the form of full-contact Kumite and Bunkai (teaching the original or alleged full-contact self-defense application of Kata).
The answer is a write-up from a lot of documentaries seen and books read, so please do not ask for individual sources ;)
When you see someone "shadowboxing" they are practicing their kata. It's a series of movements that represent blocks and strikes (feet and hands). They practice these katas to improve muscle memory. When a person is under stress, as in a real-life situation (fight), they will revert to their training automatically. This is why they perform these katas.