Why do most people think it's a bad idea to read from slides?

It's about using the right tool for the job.

There are a variety of mediums and methods to convey information (written material, multi-media, human presenter, etc.). Each has strengths and weaknesses for different kinds of information, and a role to play.

Highly technical information and tons of facts can be absorbed by the recipient looking at written or visual materials at their own pace. A human presenter can be helpful, but more so with "high level" information than extensive detail. For example, relating the gist and concepts, or providing a framework the recipient can use to organize the information they subsequently read on their own. (I'm referring to a presentation scenario rather than a highly interactive educational setting.)

If a human presenter is simply reading the slides to the audience, the slides become the presenter and the human just their A/V aid as an annoying soundtrack. Why not just have an automated slide show, or even one with a recorded soundtrack if the recitation aloud is so important? What is the human adding to the process? The biggest value provided by a human presenter is the human interaction.

The recipient's ability to absorb and retain information is tied to emotion (their interest and desire to focus on it, the personal value of the information, the novelty of it, and the strength of emotions associated with it that affect retention); it's the role of the hippocampus in learning.

That applies to any information, including technical content. The recipient needs to want to acquire the information (and have a framework for organizing it); it isn't useful to simply dispense it in their direction. If the recipient is already interested and motivated, a human presenter can either add to, or detract from, the presentation's effectiveness.

The role in which a human presenter can uniquely contribute is to engage the audience, tapping into their attention, receptiveness, and emotions to get them involved and invested in the material. That's done through the human connection, using rapport, conversational language, vocal cues, body language, movement, animation and excitement, adding tidbits of interest, etc.; focusing on and talking to the people comprising the audience, rather than reading and providing audio for the slides. Conveying high-level information in this manner is the reason to have a human presenter, and the real value they can add.

The practice of someone present for the sole purpose of reciting material displayed on slides is even counter-productive. It can make the audience less receptive to the information than just giving them the materials to read (for the reasons described in the answers by ff524, J. Fabian Meier, Hurkyl, Angew, Crowley, WGroleau, Robert Buchholz, and Superbest).