Why is Comcast high-speed Internet service so unreliable and slow as opposed to water and electrical services?

Someone correct me if I’m wrong, but my understanding is that your basic cable broadband (Comcast, etc) pile many users onto a single node, while this is not the case with FIOS. I was experiencing drastic dips in my speed from around 5pm to 10pm at night (when my neighbors were sucking bandwidth) with Comcast. I just switched to FIOS and have experienced a consistent 25 Mbps throughout the day and night.

I also hope Google Fiber makes its way to my area soon though.

I can answer your Q if you tell me where you are. I need data to answer.

@tom_g‘s answer is part of it. Cable internet connections have to be appropriately provisioned or you will feel the effects of neighbors. Comcast is famous for cutting corners on that department.
My building has both Time-Warner and RCN available, and by most reports they are both pretty reliable. Their motivation to get it right is that they know we can switch carriers whenever we want to. If you don’t have competing internet providers in your area Comcast can and will provide what they have to and no more, to minimize costs.

The other part of your question compares cable internet service with water and electrical.
The answer there is government regulation. Your water and electrical have to perform up to standards enforced probably by your state public utilities commission (PUC), and they may be publicly owned, further increasing their incentive to deliver reliable service.
Internet service is Wild-West in comparison.