You're also ignoring that the market changes frequently. If you only raised as much money as you needed for the next 4-6 months with plans to re-raise all the time, you'd have to constantly be sizing your growth plans up or down based on how the market felt about startup investing that month.
Imagine the company having to either do speed hiring or large layoffs every few months to adjust to the size of the fundraising round they were able to get this time around.
Nothing about what you're suggesting would be easier, or easy at all
Funding Round A: VC “A” invests 200M (100M immediately and another 100M if sales grow 10% or whatever)
At 6 months the company will either get the other 100M automatically (meaning they grew sales 10%) or they don’t (meaning they grew less than 10%).
Assuming it’s the later they can then do another round during which they try to get the other 100M. In all likelihood VC “A” won’t be interested (or interested at a lesser amount). They could go ask VC “B” for an investment but it will likely be less than 100M as well because they didn’t grow as much as “the market” anticipated.
Nothing complicated at all.
I’ll give you $1 dollar for your banana today and another dollar in a week when it has ripened. If it’s rotten when I come to get my banana I won’t give you the other dollar. You have your original $1 and you can still try to sell your rotten banana to another HN reader but you probably won’t get another dollar this time.If instead you have a ripe banana I’m sure you could easily find a buyer.