Early game power on Marksmen is highly dangerous as it puts power in a very simple play pattern during the stage of the game where engages are already at their simplest AND they are teamfight focused by the nature of their definition. This is all before even considering the way in which auto attacks scale with gold.
On the first point:
Marksmen are champions that depend on their auto attacks which go through a very simple algorithm to fire a targeted bit of damage at an enemy every time the cooldown is up. There's little to no counterplay outside of "don't get in their range unless you can do more damage to them" and when the only ranges that matter in a given fight are one blue team member and one red team member, there's no real subtlety to how the fight is going to play out: either you would win a fight so you walk forward to try and kill them, or they would win a fight so you back off as far as needed and hope your jungle shows up to solve the problem. In a team fight scenario there are 5 danger ranges on each team all moving around to try and jockey for position so they can isolate an enemy team member under the focus of several friendly ranges while minimizing the enemy's ability to focus any friendlies.
Contrast this with mages who have dodge-able skillshots and decent cooldowns for important abilities. This creates subtleties to a fight even in 1v1 scenarios. Compare it with Tank vs. Tank matchups in top lane which just consist of long drawn-out slapfights that are considered a downer when they dominate the meta.
On the second point:
Marksmen are ranged champions with sustained damage through repeated auto attacks that have no cost. If you change these things then we aren't talking about marksmen anymore (Graves and Jhin push the definition of "repeated," even their reload times make them feel like very un-marksman-ey marksmen). Being squishy is a logical weakness to counteract those strengths.
The basic setup for a team fight is ranged champions on each side with tanks in the middle. Ranged almost always means squishy so they don't want to be taking focus fire while the tank is seeking to disable enemy damage dealers or protect their own while hopefully surviving longer than the enemy tank. The tanks build a lot of durability so they can't be bursted down, so the total long-term dps of the team will determine who breaks the stalemate first, and Marksmen by their definition provide safe, high dps. Team fights are rarely that simple, but the result of that basic ARAM strategy is what determines which team has to make plays and picks to win and which team just has to react and maintain a net neutral to win.