I like Illustrator. Like many people, I use it in conjunction with Photoshop. It works well and has some crazy vector abilities that are unparalleled.

Illustrator is great for logo design, mocking up lots of ideas and planning designs.

Illustrator is great if you need SVGs as the final output.

However, Illustrator has far, far inferior rendering quality, so if you’re exporting PNGs, it’s a bad choice. Gradients aren’t and can’t be dithered. Antialiasing is noticeably worse.

So, it depends.

I'm the same as you. I can work super fast in illustrator when I'm actually designing -- trying ideas, iterating, tweaking. I can't imagine doing that in Photoshop at all.

Once I'm close enough to need to work with how it'll actually render, I move to the browser.

I think its because Photoshop is sort of an all-in-one tool. It can handle vectors and bitmaps so image-heavy designs could be handled better than they could be in Illustrator. I think it's also because PS works in pixels just like our screens do.

Or it could be that Photoshop was (is) like, the most-pirated software in the world.

I'm not exactly sure and I've always wondered this myself.

Main reason: people I work with don't use illustrator.

Because Illustrator isn't a web design tool. It can be used that way, the same way that Photoshop can, but to me both excel at creating assets (vector and bitmap, respectively) that can in turn be exported and composited in a UI / screen design tool. Fireworks filled that niche for a while, now Sketch does.

It's a terrific workflow, actually: I use Pixelmator for all my bitmap stuff and Illustrator for serious vector (and print) work. Then, pull them into Sketch and composite a screen design. Works great.

Note: Giving Affinity Designer a spin in the hopes I can abandon Adobe entirely.

I've noticed a lot of my students use photoshop because it's the first thing that they are introduced to; whether that was while they were in high school or maybe a intro class freshman year of college.

I switch between the two programs, and use them for different reasons. Anything that required me using the pen tool i do in illustrator. Use smart objects to transfer icons/assets between the two programs.

Wire-framing and quick prototyping I do in Illustrator. I can see multiple art board at the same time, and can have multiple size art boards which is great for laying out the same web page or app in different screen sizes.

Slicing of assets is done in Photoshop (at least when im slicing pngs - as i can incorporate Slicy into the workflow). Now that apps can accept pdf and svgs that may change, but im not quite sure yet.

Basically photoshop has better image rendering as others have mentioned. But illustrator has better pen control and allows for quicker (in my case) mockups and prototypes.

You also need to take your own personal preference into account though, so whatever you are more comfortable with will work fine.