That's actually true of pretty much all food, not just meat. The way the human digestive system is set up it's almost impossible to thrive on a raw food diet even with all the modern conveniences of selectivly bred high yield plants and intensive processing (chopping/blending). You can still survive eating nothing but raw food in a modern context, but historically it would have been extremely difficult. There's a really interesting book Catching Fire: How Cooking Made Us Human by anthropologist Richard Wrangham that proposed it wasn't eating meat that kicked off the evolution of smaller guts and bigger brains that characterizes the rise of the Homo genus, but harnessing fire to cook food in general that allowed us to decrease the amount of energy investment in food digestion (which is one of the biggest investment of energy the human body makes, after the brain) and increase the amount of energy available to our brains. This is because cooking increases the amount of energy we are able to gain from food in multiple ways.

Take a potato as an example, starting with taking a bite and chewing it. It's going to take longer and more energy to break down that raw potato into something you can swallow vs the cooked one. Then when you actually start digesting it, the cooked potato has the advantage that a lot of the longer complex starches and other molecules have been already broken down by the cooking process, while the raw potato is going to take a lot more energy to break down, lowering the amount of energy you gain compared to the cooked potato. And due to our very short digestive tracts compared to animals adapted to raw food we won't absorb as many nutrients from the raw potato anyway, whereas the cooked potato can be taken up much faster.

Of course even with cooking food eating meat was a relative rarity, with the grand majority of calories consumed coming from foraging plants in most cases. There are some exceptions including arctic peoples that had much higher rates of meat eating due to the environments they lived in, but for most hunter-gathers and early agriculturalists meat was a luxury, not a staple.

There's also other interesting side bits like the increased amount of time spent socialising around cooking fires helped with the development/improvement of communication and the intelligence that comes with that, but I've infodumped enough for right now