probably the same designer that didnt give them ST4 power swords, or grenades on close combat units, or FNP / IV saves on wraithguard/wraithlord [wraithknights can get it], or morale 10 on exarches, or A13 on the front of a falcon, or holofields or force shield on flyers or force shields on skimmers [walkers have them built in ].
neatly forcing eldar into being a 'rapid deployment' shooty army of one sort or another, or a camping army with lots of t3 troops or wraith units, im not complaining, the variety of builds is great fun to experiment with; and the cheese that is an army built around a full seer council and three wraithknights is very competitive; but the number of units that struggle even in a fluffy game is still annoying [Vypers / Banshees / Rangers / Quins / basic Wraithguard / Wraithlords / Crimson Paper Aeroplanes].
By far the easiest way to 'get' an eldar army is to nail it in close combat with something decent, the player who 'stays in the flying boxes' just needs to be blocked off objectives [tank shock has a maximum range of 12" in 7th]. The only basic eldar unit that can survive a 'rough' close combat is a buffed seer council {foot or jet}, almost everything else is T3 and dies horribly or has too few attacks to avoid being 'stuck' for the entire game.
The 'mini-rend' rule gives many eldar units some very effective shooting against small units [such as triple oblitz or 5 man termies]
but is reliant on 'luck plus' to tackle large units [like say fearless orc blobs, 15 man csm with FNP, necron wraith wall] - and most 'mini-rend' eldar weapons have a pretty short range - just soak up overwatch with some crud and get stuck in.
everyone goes on about eldar being higher/highest initiative - but at ST 3 they really struggle to do damage to T4 or T5.
with scorpions - which are ST4 in combat - just pull back and shoot - at unit size 10 and morale 9 they should run in three turns.
just remember if serpents use their op shield weapon [NB: our house rules: we have banned it] they cannot use their shield so bring one down per turn. you dont need pens. just glance them to death.