Some reasons:

We don't (yet) have flexible panels that would survive the abuse that curtains experience.

Vertical surfaces are not nearly as effective for the majority of cases in capturing solar radiation.

Windows have lots of obstructions that reduce the effectiveness of a solar panel.

Putting solar panels behind an extra layer of glass that would need to be kept pristine would be counter productive.

The voltages involved in solar panels would create an added source of danger inside the house.

Flexible conductors are very expensive (source: worked in automation). If you use cheap ones they'll fail and either the panels won't make power, and/or you'll have an exposed shock hazard.

Besides, we have way more walls than windows. Fixed panels make way more sense.

A solar panel that occasionally blocks a window would be completely useless the other part of the time.

A window permanently blocked by a solar panel is better known as a wall with a solar panel on it.

A vertical wall with a solar panel will not be as effective as one angled towards the sun, such as one on a roof might be.

Different take from everyone else in the comments. Wiring. There is something new being made popular called a balcony solar panel. It hangs off the side of your appt balcony and plugs in to an outlet. The electricity generated gets added to your appt's wiring. The problem is regulation. Most solar installs require agreements and inspections between you, the local city govt, and the utility company. I believe they're pretty popular in Germany because they set up the regulations to handle it. But many places don't want unregulated power generation. The concern being when there's a power outage, linemen need to fix stuff, and if they're expecting a line to be dead due to power being out, you can injure them by dumping electricity on the grid. My solar panels were inspected to verify when the grid goes down, they don't put any electricity onto the grid.

They're usually open when the sun's out, so pretty pointless as you couldn't charge them without being in the dark.

Why not something like this - oldschool:

I love the idea. Maybe both sides working? Or having a fixpoint at π/2.

I mean I've used a solar panel to block a window before.

But it's more practical to get cheap blinds and throw a regular solar panel on a roof. There's significantly more roof square footage than window square footage, and it's gonna get like ten times as much sun as vertical windows.

Normal panels are like $0.50/watt and will last forever. Folders are closer to $1/watt, less effective, and break down after a couple years. If you want like a set of thin solar strips in a blind form factor, that adds a lot of electrical complexity, so I imagine it increases the cost and lowers the lifespan further.

Depending on the type of panel, if any of it is blocked the whole panel will have severely reduced efficiency. Idk all the details regarding that. But if that's the case each window might have to be custom sized/wired.

I have automated blinds, and they are expensive. Solar charger was $49 which isn't horrible.

What's the point?

It would be easier to install and more effective just have the same surface of solar panel to the side of the window instead of over it.