well plants don't have nervous systems. yes, plants are living too. but so are rocks. if we get down to the essence of things, everything in this planet is energy, and definitely alive. but! if something can feel pain, evolution has given it the ability to protect itself. whether it be trying to escape, or just making a noise. when you cut broccoli, you won't hear a thing or see it try to jump off your plate. beings that can feel pain have certain defense mechanisms to either demonstrate this pain or flee it. plants can respond to stimuli, but this is merely a biological reaction to ensure survival in general, rather than the "fight or flight" response of sentient beings.
another point is that eating animals is not necessary to survival, but eating plants is. if you eat a diet of nothing but meat, you will get very sick due to a lack of nutrients. but if you eat a diet of nothing but plants, you will thrive. so EVEN if plants could feel pain, eating them is actually necessary to our survival, while eating meat is not