Why are Space Marines individually superior to Eldar? The Eldar should theoretically be better due to being "superior" and having access to greater technology, but the average Battle Brother will instead defeat multiple average Eldar before he himself is defeated. What excuse do the Eldar have for this performance?
The baseline Imperial Guard conscript has weapon skill 2, ballistic skill 2, strength 3, toughness 3, initiative 3, leadership 5 and a 5+ save. This is useful because we know what a conscript would correspond to in normal human terms; a Conscript is a regular, healthy person, recently inducted to military life, doesn't really know how to fight or shoot but has received basic training so they know how to bayonet and shoot at something (even if they're completely green). Keep in mind, the baseline Imperial Guard Conscript represents child soldiers who've been raised into soldiery from birth but are below the age of 13, and also represents mature civilian adults who've been recently inducted, or PDF who have not seen much in the way of cosmic warfare.
Now compare that conscript to the Guardsman. The Guardsman is experienced, well-equipped (by human standards), well-trained (by human standards) and well-led (you get the idea), and how does this reflect in their stats? They get +1 WS, +1 BS, +2 Ld and everything else stays the same. Compare the Conscript to the Veteran Guardsman or the elite Storm Trooper Guardsman, both of the latter get an increase of +1 WS, +2 BS, +2 Ld and the Storm Trooper gets slightly better -1 on their armour.
The child soldier, the supple civilian, the guard duty PDF, the guardsman, the veteran & the elite soldier all have strength 3, toughness 3, and initiative 3. All that changes is that the more combat effective & experienced units get better equipment, are more likely to stand and fight in the face of death and are more skilled at fighting or shooting.
But obviously all of these groups are not as strong as one another. This is because the strength, toughness and initiative does not represent how strong, tough or agile any individual is (exceptions to individual characters & generals), but how likely the race in question is going to inflict wounds, survive wounds and strike first on average. Thus it seems in comparison to races like Orks, human strength is not so different that a human civilian shoving a bayonet into an Ork or being hit by an Ork choppa is not so different as a human soldier shoving a bayonet into an Ork or being hit by an Ork choppa. Obviously the latter is more likely to do better in both cases, but the significant thing is that the difference is so marginal that the difference can be neglected - all that changes is that the human soldier is more skilled in close combat and more likely to successfully hit the Ork.
Same with initiative. A human child and a human adult are both equally slow to the Eldar, thus the Eldar will hit first.
Compare your basic conscript to the basic space marine, and the space marine gets +2 WS, +2 BS, +1 S, +1 T, +1 I, +3 Ld and -2 Armour save. Space Marines are far stronger & tougher than humans, yet only get +1 S and +1 T, which shows how the strength, toughness and initiative scale in 40k scales like the richter scale: each level is incrementally stronger and tougher than the previous; the difference between 3 strength and 5 strength is far greater than the difference between 1 strength and 3 strength.
It is worth mentioning that Eldar are not shit. Their baseline stats are superior to humans and about so-so compared superhumans, but that's unsurprising given that they're being compared to genetically engineered bioweapon superhumans. That's like saying Eldar are shit compared to Tyranids or Orks just based off of their racial stats. It doesn't take into account their synergy with their armouries (of which the Eldar get numerous weapons which are assault to them, but would be rapid-fire or even heavy to other races for the equivalent strength) and are a more psyker oriented race anyways, compared to the spehss mahrines created for more material dominance. Never rule out the role of equipment, because a child soldier with a power sword can cause more damage to power armour than an Ogryn with his trusty cleaver. Same principle with the Eldar. They don't want to get into CQC with space marines, they want to shoot them full of holes while running away.
The other thing to consider is that space marines, by virtue of their overabundant appearance in lore and table-top, can cause an impression of them being the Imperium's bassline troop, or at least very common. This belies the fact that space marines are a rare sight in the imperium, and it just so happens that the only place they're over-represented is warzones, where most of the events in 40k occur. Space marines are laughably few in number compared to the uncountable worlds full of orks, eldar, humans and so on; that they are so OP is the only reason they worth mentioning at all. Because a single space marine can go toe to toe with multiple eldar at the same time is the only reason why any other race, and the humans of 40k for that matter, notice them at all.
This short is indicative of the divide between Eldar-Tau-Human equivalents and Marine Equivalents. The latter finds walking on their opponents a viable tactic of war