The point of Pantone is to tag a color and say "This is Pantone Baby's Butt Pink" and when that image gets printed, the printer knows that it's supposed to be Pantone Baby's Butt Pink and they also have a real life physical reference to that exact color and can tell if the printer printed it correctly or not. Also when you decided on that color, you also had that physical reference in your hand and decided that you liked it, and you can expect it to be exactly that once printed.

If you just choose a color that looks good on your monitor, you'll get RGB values but you have no idea if it only looks good to you because your monitor is badly calibrated, and you don't know if it will look good when printed. Actually no matter how good your monitor is, colors will look different on a monitor than on paper. So by using an RGB color you're already starting off with an error.

Sure, a Pantone color can be approximated via RGB but no one really cares about its RGB values, because there's a much more accurate way to refer to that color: the physical swatch that you bought (at a very high price). Now all you have to do is make sure that everyone knows that you're using that exact color by name, rather than by RGB values.

Maybe "Baby's Butt Pink" is a totally different color value on one printer than it is on another. All that matters is that they know what that specific color is supposed to look like, and can calibrate their printer until they get it right. If it was just a random RGB value, no one would know what it should look like, because there is no universal agreement on what RGB(253, 229, 250) should look like in real life. One printer might print it a bit pinker than the other, which one is better?

Once you convert Pantone to RGB, your image may look exactly the same on the monitor, but you've now lost continuity with that physical swatch, and the printer won't know how it's supposed to look, and no one will know what that RGB value is supposed to look like in the end once it gets printed.