'why not' ?
Actually the requirements differ between Mass Effect and Mass Effect 2. In Mass Effect you can only recieve the squadmate's achivement if you complete 75% of the game with them. However this requires that you have the squadmate with you for all the main missions upto Vermire. For the humans you'll need to take the survivor to Ilos and that survivor must have gone with you on the other main missions. On top of that you'll need to complete 75% of the side quests avalible by talking to others, Admiral Hackett comm'ing you when you go to other places, and actually exploring the planets themselves. In order to recieve all the achivements you'll have to make at minimum, 3 playthoughts. Since you need at least one playthrough in order to unlock Insanity mode and make two playthroughs for the game achievements that shouldn't be a problem. If you don't have the patience for it you're S.O.L.
For Mass Effect 2 the achivements differ a little. They require you to recrute and complete the Loyalty missions for each Squadmate. This is possible in one playthrough and is required in order to survive the suicide mission as well as get that achivement. You'll still need to make at least one playthrough before unlocking Insanity difficulty though.