Mushrooms are about 90% water, but the non-water part is indeed roughly half protein.
How many oz of mushrooms would one have to eat to get the same amount of protein?
Edit: legumes are more practical, green peas give you 2g/oz and peanuts can be 7g/oz shelled according to google, (though they are different proteins than the beef). Mushrooms are good for texture and some like shitaki can give the umami taste, but they aren’t a nutritional meat substitute on their own.