Ramp is the bane of Jund decks.* You go up in mana quickly and then blast them off the board with a big, very powerful threat that they can't counter and can't answer effectively. Since they lose if you get to cast the big threat, their only options are 1) kill you first, 2) stop you from ramping and 3) discard your payoff. Unfortunately for them, none of these work well. They are not an aggro deck so they can't kill you quickly. Land destruction typically costs 3 mana or more, and discard cannot stop what's on top of the deck.
Jund is very good at navigating to a state where both players are empty-handed and topdecking, but in this scenario, ramp is favored simply because the threats they can topdeck are better than the threats Jund can topdeck, and every one of their threats is worth several of Jund's cards.
To illustrate the theory we can look at two of Jund's worst matchups in Modern.
Imagine the Tron player goes first, and they play Urza's Mine & Chromatic Star.
On their turn, the Jund player goes Thoughtseize. (If they don't have Thoughtseize they are in even more trouble because they cannot stop the incoming Urzatron - and hence 7 mana - on turn 3.) They see a hand of Ancient Stirrings, Urza's Tower, and Sylvan Scrying.
Note that regardless of what they take, they are backed against a wall. If they take Ancient Stirrings, then the Tron player will play Sylvan Scrying next turn using the Chromatic Star, get an Urza's Power Plant, and Tron will be active on turn 3. In other words, any topdecked payoff (Ancient Stirrings is almost a payoff as well) is lethal. There is no good way for Jund to answer a resolved Wurmcoil Engine or Karn Liberated unless they already have an overwhelming board position. On the other hand, if they take Sylvan Scrying, then Ancient Stirrings still gives the Tron player five looks for Urza's Power Plant (or Expedition Map which will turn into Urza's Power Plant).
Note that a key part of the equation is that Urzatron plays threats that catch them up if they're behind on the board, win the game if unanswered, and are not easily dealt with by Jund's set of answers. Karn Liberated & Ugin, the Spirit Dragon for example just win the game if cast (Jund cannot counter them). If Jund has threats on the board then they will be eaten, and Jund has no way to kill them immediately from an empty board. Wurmcoil Engine is similar: it stabilizes the board and is too large to kill with Fatal Push or Lightning Bolt. Terminate, Kolaghan's Command, Liliana of the Veil's -2, etc. can answer it, but that still leaves two more pieces.
Imagine the Amulet player goes first, and they play land into Amulet of Vigor.
Now the Jund player is once again in trouble. Next turn, the Amulet player can easily go bounceland (e.g. Simic Growth Chamber), Azusa, Lost but Seeking or Dryad of the Illysian Grove, replay bounceland, Sakura-Tribe Elder. On turn 3, the Jund player will be up against the likes of 6+ mana, making any Primeval Titan lethal. (For anyone unfamiliar with the Amulet Titan combo kill, the Titan searches up Sunhome, Fortress of the Legion and Slayer's Stronghold. The lands give the Titan haste, it gets to attack and search up even more lands, and the opponent dies immediately or at most one turn after.) The Jund player's only chance is to discard the Titan, but even if they discard any Titans in hand, they can't stop their opponent from topdecking another - and opponent has at least 10 copies of Titan in their deck (counting Summoner's Pact and Tolaria West transmuting for Summoner's Pact).
Even if the Jund player stops the Titans, Valakut, the Molten Pinnacle enables a different kill that, because it comes from a land, Jund has trouble interacting with.
These ramp matchups are why Jund decks usually have land hate like Fulminator Mage and Alpine Moon in the sideboard. However, one can only dedicate so much space to fighting ramp, meaning Jund might never draw the sideboard card. Furthermore, these land hate pieces don't win the game if they resolve, they only stall the ramp deck. Finally, the good land hate spells are expensive. Fulminator Mage hurts the ramp deck pretty badly, but it costs 3 mana, too late if the opponent is on the play. Even if they are on the draw, using your turn 3 to cast a land destruction spell means you're not adding pressure to the board. Alpine Moon is even narrower, and even fixes the opponent's mana.
I personally found this article to be a hilarious illustration of why Jund is disfavored against ramp. It took a 2020 Standard Bant ramp deck and pitted it against a Modern Jund deck. Bant ramp won easily, in spite of drawing from a much smaller card pool and playing strictly-inferior (by Modern standards) cards like Shatter the Sky. Heh.
*Note ramp is not the same as control. Ramp decks focus on cheating more mana into play and then resolving a game-winning spell. Control decks take things slower and aim to answer the opponent's threats one-by-one. Jund is far better against control decks than ramp decks.