Profit earned is all that counts in the end. Note that I'm not saying that day trading is profitable for everyone or it is a losing game for every trader out there. But it simply can't be gambling if you accept that trading as such is not gambling. There is no other choice because all price growth starts small, even profits from long-term investment are made of small price moves. Capitalizing on the volatility which results from these small moves allows you to earn higher profits because there is no growth without corrections, however small those can be. Add to this that you can't always be right in your assessment of the future growth and even more so in respect to the extent of it.
First of all you talk about capitalizing on short term volatility like it is easy to do, and like people are capable of doing it consistently. When you buy an asset with the intent of trading it intra-day, how do you know it is going to go up and not down? You can lose money as easily as you gain it. There is no way to accurately predict price movements on this small of a scale CONSISTENTLY. Day traders will lose just as often as they win over a long enough period of time, and because of fees, this will end up putting them on the losing side overall.
Also, profit earned is NOT all that counts. If I am day trading in the stock market and earning 8% ROI in a year, but the S&P500 returns 12%, all of my work figuring out what to sell and buy and when, was just a waste because I could make more profit by just holding an index fund. Same with bitcoin - if you day traded bitcoin in 2017 and made a 200% return, you have failed because you would have made more money by just holding bitcoin. All of that day trading not only failed to help you make money, but it resulted in a massive amount of wasted time and wasted potential profit.
And regarding your comment about not always being right about future growth... it's true that you won't always be right, but you can be right the majority of the time. You can certainly be right MUCH more often than day traders. If you put in the time/effort/research for long term investing, you can get to a point where the only time you are wrong about the long term prospects of a company is pretty much when some unpredictable catastrophe happens, or some information is being deliberately kept hidden.
And you completely write off arbitrage which belongs to day trading as much as day trading is concerned.
You would need to use trading bots for that and take into account trading fees obviously. Ultimately, it all comes down to how efficiently you allocate your trading capital so as not to lose profit on bigger price moves. It looks like you think that day trading excludes any possibility of "seeing a long-term value in an asset". This is another misconception, of course.
Day trading by definition does not involve looking at the long term value of an asset, because the long term value has zero impact on the price within a given day. Take Snapchat for example, which had some great days after its IPO, spiking from $17 to $27 per share, even though long term it is a garbage company.