Yeah the other odd thing is that for some reason opening the door is using a skill in 4e but not in the 5e comparison... which would definitely favor the fighter.
The Fighter is using the skill in the comparison. A DC 15 check by an 8 Str Wizard untrained at 1st level means the Wizard needs to roll a 16 or 25% of the time. The 20 Strength, 20th level(+6 proficiency) Fighter has a +11 and therefore needs to roll a 4, or 85% of the time.