CBA to read everything, but:
Yes, it can be annoying, but it is also a part of the game-mechanic. By forcing you to do the easy part over and over again, you are farming souls ( as long as you can safely get back to your corpse), because sometimes you just need to level up to keep going. Also, the bosses are rather easy. Gaping dragon done in two tries (because i failed the first time), Quelaag down to 75% first try, from then on i knew the moveset and what to do (still failed for two hours though). Also, having more checkpoints would take away from the experience. It is supposed to be depressing, it is supposed to make you angry when you die. E.g. if there where checkpoints every 5 enemies (right now it feels like checkpoints every 10 enemies), then i would not understand the depression some of the characters have.
TL;DR : It's part of the game mechanic (farming souls) and atmosphere.