People opt for so many trading pairs because they feel they might at one time land on a gem that can easily go x10 in a few days. Of course this happens at times but not always. To answer the question, I think it just depends on one's strategy. For someone trading conservatively, focusing on a few trading pairs is much better, especially high with high marketcap coins couples with higher time frames is the bomb. The only disadvantage is maybe lower PnL over a long period of time but the risk of losing a trade is way so low.
I think it was the investors are the ones who does that and not the traders because when you are trading, you don't wait but you need to flip your assets from time to time. It is still possible to make huge amounts of money through trading because the profits that we can constantly get will just stack up as long as you don't spend them yet. I agree with you that trading with a few trading pairs is better because it can give us more focus. Others think that having more trading pairs can also give them more profit. That was true as long as they are able to handle it and we can believe it better if they are already doing this before and they have proofs to show with us. Choosing high marketcap coins is another positive thing because we know that it was the low-cap coins are the ones who are high in risk.
All altcoins are almost the same therefore if you can master two altcoins, you can also be able of trading other altcoins.
All are not the same but almost? Yes. I think only 10 percent of altcoins are there that are legit. With all the the sea of altcoin there must only be one altcoin that is worthy to get and that would be ETH. There is a saying that two is better than one but we can take BTC as a sample here. Many people say that BTC must be the only coin that we must hold, though if we can also consider an alt to our holding then we can also go for ETH.