Think how space war in Trek is fought. Compare with naval warfare, especially of the Horatio Hornblower era. And please forget all about land warfare, because it does not apply here at all.
Planets sit far apart. How far? Traveling even a single lightyear takes plot time, as seen this week. Within plot time, an attacker can complete an attack aimed at destruction: messing up a planet down to the mantle was supposed to only take five hours in DS9, and glassing the surface actually took just seconds, with enough starships at it. 23rd century guns might not be quite as powerful, but they need not be much weaker, either.
The important thing is that the attacker can arrive in sufficient numbers to complete the attack in allotted time. He gets to choose the numbers, and the target. The defender has to guess: do I move my ships to this planet or that one, in wait of the attacker? If I don't move all my ships, the enemy will outnumber me; but if I move all of my ships and choose the wrong planet, the enemy wins.
The attacker wins, then. Assuming the defender is blind. But Starfleet is not blind - the Discovery all on her own was able to track down ISS Shenzhou across random but necessarily considerable interstellar distances (especially given how Burnham this week claims the Terran Empire reaches far beyond the area explored by the Federation). So the defender can and will move his ships to counter the attacks, in a giant game of chess.
Now enter cloaks, and the defender automatically loses. In this case meaning, the UFP automatically loses, as it is the defender, and cannot be the attacker, as its ships aren't invisible.
But a single attack sortie of the suicide sort is always available to the Feds, restoring their local numerical superiority through a brute-force approach. It's natural for them not to think of doing that, I think. Not unless there's a reason to think it will win the war for them.
Basically, Starfleet is learning how to fight like Hornblower's First Sea Lord after having gotten used to oceans-wide satellite coverage. The learning curve just isn't steep enough. But the guns and ships involved remain at the relative Hornblower level. Setting fire to an undefended Caribbean island's forests is doable. Bombarding a fortress to submission is doable. Conquering an island is a chore. Fighting one's way to an island defended by ships is not worth the bother. And thus we have these brave, foolish or cheap attacks by the Klingons, with control of empty and worthless "seas" flowing back before it again moves forth.
Timo Saloniemi