Gonna Copy pasta from another thread that I've said this on already. Note that I am not quoting this thread's OP but this thread's
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The potential gold you can get from Cull is 100+300+180= 580 - 450 = 150 G. So you get 150 G if you cs perfectly at 11 minutes and if you forsake the extra stats of the a Doran's Blade. 150 G. That's practically nothing, but that's your **potential **gain.
The potential gain you get from a doran's blades is 450 - 180 = -270. Meaning you take a gold loss. Also learn math 580 - 450 = 130. meaning using only what's listed only the item's, you are up 400g from your opponent by taking cull and finishing it.
On the other hand, if you opt to start a Doran's blade instead, you get 80 HP and +3 AD and 3% lifesteal that greatly compliments the new mastery changes and can scale. These small stats that you gain by choosing Doran's Blade instead of Cull are vastly, immensely, and exponentially more valuable than the measly 150 G.
Cull gives 5 AD and 3 life on hit. Now early game 3 life on hit is more effective than 3% lifesteal as you need 100 AD to break even (if they have 0 armor) for lifesteal compared to life on hit. In addition cull gives you money to set you up for the late game. Now in the bot lane it's more efficient to take doran's blade as you remain in lane longer (and thus have more gold and time to reach that 100 AD). However cull is aimed more at top and mid laner's who tend to have more farming efficiency and tend to back earlier to buy. The HP of doran's in nice, but with the fact that cull has superior early sustain, on top of the sustain that top laners normally have this makes it a better item for most ad top laners and a decent item for certain AD mid matchups.
Most people hang on to their Doran's for a long time. That's how valuable and gold efficient it is. The lane phase power that Doran's Blade offers is too much to ignore. Why would anyone want a potential +150 G when they might lose just as much or even more gold because they were denied CS in lane as a result of Cull being weaker than Doran's Blade?
Another big reason most people hold on to Doran's Blade for a long time because if they sell it, they are taking a loss in money value of items. So even though they may be up by 300 gold on the other laner in terms of gold they've made, if they sell doran's blade to buy something they're just putting their actual item value at the same level as their opponents. Also they hold onto it because they tend to have room for it in their inventories and it doesn't make sense to get rid of it when it doesn't cost them anything to do so. (Same reason people hold on to cull after it's passive has finished, except you don't feel bad for selling it, unlike doran's blade)
If cull could upgrade, however, then it might actually be worth while. It makes sense for it to upgrade into Vampiric Sceptre both stat wise and visually even. So, why doesn't it?
Riot's done plenty of posts of this (you can check old forums for them), the reason starting items don't build into anything is because they are extremely stat efficient at the cost of losing gold when you sell them later. Cull merely provides it's stat efficiency in the form of giving you gold.