Answer:
There are many reasons for this. First, it's because the link between the written and spoken language is very weak. While in Spanish you can mostly read what you can understand when spoken and you can write what you can say (bar some minor spelling problems), in Chinese the two are more or less separate.
Second, the way Chinese characters represent sounds is complicated and requires much more than learning an alphabet. However, the process of learning to read and write can be made a lot simpler than it first seems. Learning 3500 characters is not impossible and with proper reviewing and active usage, you can also avoid mixing them up (this is actually the main challenge for non-beginners). Still, 3500 is a massive number. It would mean almost 10 characters per day for a year. Added to that, you would also need to learn words, which are combinations of characters that sometimes have non-obvious meanings.
Sorry that it's long.