Is a telescope difficult to make? Does glass have to be polished and shaped very precisely? Or is a device using two or more lenses to magnify things just not obvious?
Ancient glass was opaque or at most translucent. It was also often full of bubbles. Glass was not even suitable for windows until the first century AD, and even then it was as a means of letting some light in, it wasn't useful for looking through.
Lenses were made in Mesopotamia, but using quartz crystal instead of glass, and were costly. Nero is said to have had glasses made from emerald. Only an emperor could afford such luxury.
Transparent glass and the development of techniques to make flat planes of glass date from about 1000 AD and was developed in Venice. This is just an example of slow evolution of technology. Making transparent glass requires a pure source of silica and soda ash (or potash, but that comes with other issues), chemistry to supply various oxides that improve transparency, more advanced metallurgy, better furnaces and so on. Transparent glass is said to be the discovery of Angelo Barovier in the middle of the fifteenth century.
Glass had developed to a quality that made is a suitable substitute for quartz in lens making by the 16th century, but it was still secret knowledge that few held. The process of grinding and polishing glass to a suitable degree takes time, and needs the development of fine abrasives (or you just end up with lens that is so scratched to be opaque again).
It was only after the printing press had been developed that the theory of lens-making became widely known, and the first telescopes were designed. Even then, the idea of using a telescope at night wasn't immediate. The people making lenses weren't the same as the people who tracked the planets.
Lenses for telescopes need to be an order of magnitude better in quality than lenses for correcting long-sightedness. The glass does have to be shaped and polished very precisely. The technology and skills required for this were not fully developed and shared before 1600.