Not taken into account in your comment:

• TTK if Tundra misses or hitmarkers their first shot: increases by ~1000ms

• TTK if XM4 misses their first shot: increases by ~20ms

• TTK if Tundra hits a headshot: The same as a bodyshot

• TTK if XM4 hits headshots: Improved beyond the point a sniper could compete in your example

• Tundra caught off guard at close range: risk a noscope and hope to get lucky or attempt to switch weapon

• XM4 caught off guard at close range: start reliably hipfiring immediately as you transition into ADS

If you compare weapons in the most basic way possible like you did, you've found that they have approximately equal time to kill if both players are in a neutral situation and play perfectly. This is extremely uncommon in practice; you frequently come up against more than one enemy at once for example and even the best snipers only have around 60% accuracy. Other weapons are far more consistent and forgiving.

Just compare peoples' stats. Pretty much every player performs worse with snipers and has a better K/D or SPM with other weapons. Some of the top snipers in the world like Scope, Spratt, Obey Roy, etc typically only have around a 2 K/D and 1000 SPM. There are random players on this subreddit boasting about stats better than that, and a cursory glance at Twitch shows unknown "pubstompers" advertising 4+ K/Ds.

The truth is that aggressively sniping is a high risk, high reward playstyle.

They're annoying to die to because they kill you "instantly", while dying to another weapon lets you "feel" the hits for a moment before you actually die, even if you didn't stand a chance there either. But ultimately you can counter them very easily by just jumping. Use almost any movement techniques at all and you'll beat them. If someone is beating you with a sniper, they'll beat you even harder with another weapon.