Firstly, because it's a card game that can easily be detached from randomness. Generally, in any other game of this kind, it's challenging to assert that luck is not the most crucial factor. In poker, while you can experience both good and bad runs, your skills make the difference.
And secondly, personally, I find it very enjoyable. If you want to improve and delve deeper into the game, there are always more things to learn and enhance. Besides, it's quite challenging and provides economic rewards as well.