First the action and barrel is fitted in a one piece stock, this means you can free float it or put pressure on the barrel to make it shoot better...
Next, the trigger is fully adjustable, because of the design of a semi, this isn't possible...
Because of the above, bolt actions are usually more accurate than semis...This is why those that shoot long range targets and snipers use bolt actions...
Reliability, a bolt action won't jam like a semi can...
Mounting a scope is more solid and it will be more reliable because of the action of a bolt action...Most bolt actions are more balanced and lighter than a semi...Lever actions are cute but for some of us they are simply too short and don't fit us well...