I know how to use both to make basic web apps, and Flask has just been an overall better experience.
1. Flask is easier to get started with. Make a file, type like 7 lines of code to create a route, and add stuff as you go. In Django, you are overwhelmed by a bunch of files with no explanation to what they do.
2. In Flask, you have control over your app and know what it is doing. Django users claim that giving control to Django saves time, but in my experience it makes things 1000x more confusing than it needs to be. For example in flask, you actually make login cookies and stuff to handle logins. In Django, you pass it to this built-in user model and some black magic happens, then you have to keep googling how to change placeholder text and what forms show up.
3. Handling forms in Django is atrocious imo. Which would you rather do?: Type like 4 lines of HTML to make a form, or like 100 lines of django form trite and all kinds of super().__init__ widget.TextInput workarounds just to change the css on form? After I write Django forms, I look at what I wrote and it's incomprehensible.
4. Unpopular opinion, but Flask documentation is so much better than Django. If you wanna figure something out in Flask, you go to the docs and it's like "okay type this stuff and it will do the thing". If you wanna figure something out in Django, it's like "subclass the user object args to init the method to the routes" when you're just trying to print out some text. It's also badly organized. What is the difference between "try a tutorial", "dive into the documentation?" and "Django overview"? What order does any of this go in?
5. Flask can make pretty decent APIs ootb, and flask-restful just seems to be more documented and popular than drf. And do we need this much bloat to make a basic API that can be spun up in like 5 seconds in vanilla flask? Writing an API in django is like playing pool with a battering ram.
6. Flask generally leads to faster applications because there isn't a million overcomplicated things going on behind the scenes and it is a smaller library.
7. Finally, Flask just gives you a better understanding of how web apps work and knowledge that can transfer into other frameworks. For example, flask uses flask-sqlalchemy. SQLAlchemy is a massively popular ORM that is used in other languages and libraries, so using it in flask gives you a head start when learning other frameworks. You also learn how cookies, sessions, SQL queries, etc work at a fundamental level.
Django all around just reeks rushing out a bloated application to save time for companies. I actually enjoy writing apps in Flask, and interestingly I'm seeing more jobs appear for it nowadays. Django isn't outright bad but I don't see any reason to use it over Flask except for it being more popular.
Finally, a lot of the time saving stuff you can do in Django can be done in Flask. Want a basic app structure ootb? Just make a boilerplate and copy/paste when you want to start a new project. Want an ORM and api stuff out of the box? Just add flask-sqlalchemy and flask-restful to a requirements.txt and write a shell alias to install it all at once when making a new project.