Why’s picking a handful of ETFs I am bullish on in the long run and reinvesting in whatever is most red, worst than an allocation strategy?

At the risk of sounding silly; why is picking a handful of stocks/ETFs that I am bullish on in the very long term (a few business cycles, not just a bear/bull run) and dumping money on whatever is most low at the time, a bad or not very suggested strategy?

I understand this may sound vague, so I will present my thought process clearly while providing context. Firstly, to put things into perspective, I am a 19 year old student with limited experience, hence excuse the rather simplistic line of thinking.

So being based in Australia, I have narrowed down my invesment allocations for my long term personal finances into these four:

VGS – Vanguard Global Stock (70% Shares in the US, 7-8% in Japan)

VAE – Vanguard Asian Emerging stock (Excluding Japan)

VAS – Vanguard Australian Shares

NDQ – Nasdaq top 100

So I am bullish on all of these in the very long term and starting two months ago, have been able to allocate $1000 a month. My problem came from trying to mess around with the allocations and all that jazz, after seeing different allocations recommended from different folks. This became especially a bitch since I can’t buy fractional shares on my brokerage.

Now I’m considering this line of thinking; ‘hey, I am bullish on all these ETFs (plus NDQ) in the long term, don’t see them fucking up my money too much, why don’t I whenever I have the chance to deposit the 1000 bucks a month, allocate it in whichever out of the four is most in the red. If there’re multiple in the red, make a judgement call, if all in the green, make a judgement call, etc etc’

I always wondered why folks stick to a strict allocation when they can choose to think of whatever is most down as an opportunity to buy, since they are bullish in the long term anyway.

I am certain there is something I am missing here, perhaps lack of diversification? Well that’s when the judgment calls come in. Perhaps the psychological strain on dumping money on red not sounding as easy as it does right now? Then yeah fair point, no clue on my end, I have no experience.