investing all at once will only increase the risk, but if you are here for long term hodling, then there’s no issue with that. However, if you aim to seek for short term profits, even with a small budget you can start accumulating bitcoin. The price may be expensive though, but when you buy through DCA, you will not feel the burden of having to spend a huge amount just to have bitcoin.
Nobody should be investing everything at once because even the big Investors that have billions to invest don't invest like that. We have to learn to invest like the big boys are doing. Small amounts makes you to learn from your mistakes so that when you want to invest further, you won't be repeating the same mistake that you were making when you started trading. Trading gets better with time if you're focusing on the right things and learning from your mistakes but not many traders learn. They keep trying new strategies in the hopes of getting quick money but it all fails them and they quit. Some get lucky but it doesn't happen to many people therefore we get lots of quitters for traders.