What makes a customizable website better then a pre made website?

Unfortunately, you are going to get a lot of opinions that may not answer your question. Do you just need to "get the job done"? Doing anything custom is going to be far more work. If you plan on being a web developer (as that might be your goal) and you see that you could market your skill for a few small business, well okay. Might I suggest this stackoverflow link here. The comment was in 2011, but the essence of his remarks remain valid today.

I used Moonfruit to create a site for the company I worked at, and a site for my bosses side company. I was elected to do so, as I was the only one in the company, who was handy with a computer as they would put it.

At the start it was ok, nice and easy to get used to, with some good templates to play around with. Then when problems started, I would contact their customer support, and would have to wait 2/3 days until the issue was resolved, often their staff were no help whatsoever.

They recently release there new V6 HTML5 web-builder, it been open for over a year now, any many of the features found on their previous versions of the web-builder aren't even complete, they've only just released a beta of the contact form widget.

Then there is the problem of limitations, you are limited to the simple tools supplied to you by the CMS.

It was these continual problems that slowly irked me into looking into maybe learning to create the sites myself.