There are a lot of different factors that can cause guitar slipping. We can practice standing up for sure, but if we’re taking something seriously for several hours and we have to look at the fretboard, it’s highly straining on the neck to do this for long periods of time especially if you’re having a guitar’s weight hanging on your neck or if you’re trying to follow sheet music, tabs or chord charts or even the school lessons. You don’t want to be leaning over doing that with the weight around your neck. All of this would be very inconvenient to follow when standing up. Sometimes we like to sit down so we can focus on stuff.
Some of the factors can (I hate to say it for us girls) big thighs. I’m a chunky girl and the curve of my thighs makes the guitar slip so badly if I rest it on my right leg. This is why I play classical style by putting my left foot on a footstool and resting my guitar on my left leg. Another one is the fabric of your pants. Things like leggings or workout pants, loose fitting pj pants can make the guitar slip badly because of the thin/slick fabric. If you’re playing on a guitar with a larger body like an acoustic, they can also be prone to slipping.
Another factor is if your chair is too high. It will make your thigh slant downwards too much which will cause slippage.
All in all, my advice would be to get a foot stool. If you don’t have one you can use something like your guitar case laying flat. I do agree it’s crucial to spend practice time standing up but I like to do that after I’ve gotten something down a little better. Hopefully this helps!