If you limp, how do you win a big pot?
With less in the flop due to everyone limping, you're almost never going to win a sizable pot unless your opponent hits the flop very hard, but you hit it harder. That is a setup hand, or a cooler, depending on which side you are on, and you can make more money if you had raised pre-flop in that situation anyways. When you raise pre-flop, it gives you more options. You take control of the hand. On the flop, you have the choice of continuation betting when you miss, double barrel bluffing to show your aggression, taking a free card if you are on a draw as most players check to the pre-flop raiser, or value betting significantly more when you do hit. In a limped pot, people are more apt to play fit or fold, meaning only if they hit the flop hard will they continue seriously. It is nearly impossible to stack a player in a limped pot. In a larger pot, poker players will fight much harder to contest it.
If you limp, you can lose a big pot.
You limp a mediocre hand and are unsure of where you stand because the hand ranges weren’t defined well with everyone limping into the pot pre-flop. Let’s say you limp J,8s and flop a flush. Another player in the blinds may have called too with Q,9s that they would have likely folded to a pre-flop. Essentially, limping will put you in tough situations when you hit big hands that are much more routine if the pot had been raised.
The more often you're raising pre-flop, the more likely it is that your opponent will get frustrated and make a stand against your frequent pre-flop raises at the wrong time, or spew chips off by making a hero call later in the hand. As you notice people making plays at you, or calling you lightly, react by either firing more barrels at them on the turn or river or folding the suited cards yo speculated with pre-flop. The great thing about a raised pot is that you don't need to get to showdown to win. You can with two ways. You can bluff them off the hand, due to your aggression or if you do hit, your hand strength is disguised. Limping and calling just bleeds money. Why limp 55 UTG and then call a raise from BTN, only to c/f a J high flop that you could easily have won by raising pre-flop and then continuation betting?
One example of limping with purpose when you have a small to medium size stack is if you have an aggressive player still to act who you suspect will raise big on your perceived weakness. You can then re-raise them big when the action returns to you with the likely best hand. The difference being that you had a plan in mind of setting up a player, not settling for the weak limp into the pot.
By raising pre-flop, you're creating +EV situations where none existed before. Couple this with solid post flop play and you will already be improving. Limping is just asking for good players to pick on you.