It’s really not, especially for high end models if you are stressing your hardware and it’s not an awfully thermally limited design you’ll drain the battery even when you are connected to the charger.

You have 60W chargers usually for systems with max power draw of the CPU+GPU that can reach around 200W peak and in the mid to high 1XXW sustained if you have a DTR/high-performance “mobile” CPU + dGPU.

Even if your laptop comes with a 90W charger it’s still a problem because it’s not enough to power a 65W series H CPU and a high end GPU that draws another 80-120W.

Most laptops will always use the battery directly and trickle charge it due to the power draw.

Power(hungry)ful modern gaming/DTR style laptops can use both concurrently, heck older gaming laptops used to come with multiple power supplies that were needed simply because even the battery wasn’t enough at the time to serve as a buffer, so you had to plug in 2 power adapters to power the laptop fully.

Even without this constraint having a single power path is cheaper and simpler to design so most manufacturers just only draw from the battery and have a separate charging circuit that charges it without having to add power balancing and switching circuitry.

IBM thinkpads used to advertise that you can switch the battery while the laptop was plugged into the charger most laptops of that period didn’t support this either.

If you go even further it was even more of a headache because older laptops used LiPo batteries which were even more finicky.