Dear, there are many reasons behind buying or investing only in bitcoin is that it is only the strongest digital currency out there. If we talk about the current scenario Bull run is on the way and is halving is too near where bitcoin can make some price correction which will give buying opportunity to the investors it is saying that bitcoin will reach 170000$ in this bull run because there is many positive news in current time which bullish to investors to buy aggresively. DYOR it has less risk as compared to other altcoins.
It's normal for everyone to say, as you say, that buying and investing in Bitcoin is the best choice because Bitcoin is well known and very popular with all investors as an investment that always provides the best for its holders. It's true that the halving period is just a matter of time, and it is a sign that bullish times are coming. and bitcoin will increase and altcoins will move following bitcoin's movements, so of course this opportunity should not be missed if you don't want to just be a spectator where those who have investments enjoy the profits.