Why people usually read Upanishads as a simple, easy and at the same time a profound way of understanding Divine truth and gaining more insight into the true nature of Self/Cosmos.

It is believed that there is no supreme text than the Vedas and the Upanishads. These books are believed to contain words from the Supreme Divinity itself, that have been transcribed by man after long penance. Thus, these books contain essential truth (the very essence) about one’s inner self and the knowledge about the origin and workings of our universe. A person who is willing to know about this truth and wants to know what rules the world and how can he face every challenges in life or any other question which he may be pondering every day in his life will find an easy resolution by reading these sacred texts. 

Science has now agreed upon the point that most of its so called modern day "discoveries and inventions" are already referenced in the Vedas and Upanishads. E.g. being concept of atom, space technology & travel, nuclear warfare, just to name a few. Major Upanishads play a central role in developing this knowledge about truth on the nature of Atman and Brahmana. Minor Upanishads give an even more detailed insight about the same. Every knowledge that exists on earth, explored or not as yet, has already been detailed out by the Supreme God , in the apurusheya Vedic texts. These Upanishads serve as an easy mode of understanding this essential and existential truth as revealed by the Divinity to ancient Vedic Seers – who in turn conveyed it to their disciples and then to common man in an easy and understandable way.