You literally get 25 attack damage for 600 gold when you upgrade to hydra having the 2 items that goes into it, where if you were to buy that 25 AD it would cost you 875 gold the 3 items go into trinity, when you upgrade it costs 3 gold only once you have shiv zeal and phage and you don't even get a state increase just makes it more compact and opens room...
MrPerson101 votesYou literally just answered the very reasons why they do it.
Compacting the spaces = gold. How would you like to be forced to buy 6 item slots of stuff to get the effects of 3: 
?
PedramCarter1 votesIt's not fair though you don't understand, I mean trinity force is fine, shouldn't hydra be the same too? seen as you get extra health regen + life steal + attack damage?, either they need to balance other items properly so for example increasing vamperic damage or nerfing hydra (which I don't support) it's not fair only other champs to be able to get all the other extra stats from hydra + the 25AD just for price of 600.
MrPerson101 votesTriforce you get all the stats combined, exactly as it is, and costs you next to nothing - agreed.
Hydra you do get an extra 25AD and 4% lifesteal from the upgrade and it costs 600, true - but by the same token, 
makes up for that with it's high combine cost of the ingredients (875+360+180x2=1595, combine cost of 305)
Now take that difference, 305 + 600 = 905g
cost 875, difference of 20g - assume that's ~ the cost of 1% life steal, you're really only getting 3% lifesteal for free.
PedramCarter1 votesThat does make sense but it's the same thing for trinity though there's items that go into items zeal and phage and shiv so it still no divided accurately technically. because if you're racing damage against a lane ans you have those 2 items against someone who needs to or wants to build blood thirster they'll lose (I know what I said isn't accurate and you don't need more damage to win fights, just an example)
MrPerson101 votesEssentially the point I'm trying to make is that with Hydra you're combining materials for extra effects, with Trinity you're not - you get exactly what the 3 components have, summed into one item.
PedramCarter1 votesMakes sense I guess :D
JustMyBassCannon1 votesAn item's cost-efficiency is based on the lowest-tier building block for the stats it possesses. For example, BF Sword is 116.1% efficient compared to Long Sword (the base of cost-efficiency for AD).
Ravenous Hydra grants 75 AD (worth 2700 gold), 100% base HP regen (worth 360 gold), and 12% Lifesteal (worth 660 gold), being worth a total of 3720 while costing 3300. We can't really apply a price to the unique passive/active effects, but it's worth mentioning that it has those effects as well.
Trinity Force grants 30 AD (worth 1080 gold), 30 AP (worth 562.5 gold), 30% AS (worth 900 gold), 10% Crit Chance (worth 500 gold), 250 HP (worth 666.67 gold), 200 MP (worth 400 gold), %MS (which doesn't have a solid base, as the value fluctuates from costing 0 gold for 5% on Zeal to 197.5 for 5% on Aether Wisp), and the two passives--a hyper Spellblade and the Phage Rage MS steroid. Worth a total of 4109.2 gold while costing 3703 gold.
Hydra is ~113% efficient, Trinity is ~111% efficient, before accounting for their passives/actives that can't be valued with gold.
I'd call that a pretty fair spread value. Hydra only offers one directly offensive stat while Trinity offers four, which means that Hydra has less overall power but it also has more focus.