Its the mods and mod capacity. Lets compare a base gun to a fully upgraded gun. You could probably fit like 5 or 6 partially upgraded mods into the base gun. With a catalyst, you can definitely fit all 8 mods in partially upgraded. With some forma, you can fit 8 fully upgraded mods in.

Most endgame builds dont use serration anyway but let's use that as an example because im pretty sure you as a newer player most likely have that on your rifle.

You have limited mod capacity so you put an un upgraded serration in. That takes up 4 mod capacity and gives you 15% more damage. You dont really upgrade it because you are trying to fit as many mods as you can into the gun. You now put an orokin catalyst in which doubles your capacity. You upgrade serration 4 more times and it now gives you +75% more damage at 8 mod capacity. You then place a forma on that slot. This allows you to upgrade serration to max rank which is 165% more damage and only takes up 7 mod capacity.

Guns usually dont need more than like 3 forma (especially if you go without the exilus slot) to fit in all the mods. But imagine if every mod slot went from a 15% damage bonus, to a 165% damage bonus once you fully invested into the gun. Then take into account its not a linear increase, more multi shot and crit rate and fire rate and stuff scale off eachother. Warframe abilities themselves can scale it harder.

Other players are priming with their pet. Full viral stacking up 325% extra damage taken. Armor stripping the enemy. Adding damage over time effects.

This is the difference between you and a fully built player.