That's why they calculate it as year-end finals, isn't it? So that they don't get into minor discrepancies about gains and losses along the way. There's no way that they can charge you for every gain and ignore losses. That's just not a sustainable business.
They don't ignore losses. The problem is when you don't make provisions towards tax (you don't put fiat money aside) and you wait till the end of the tax year. You're exposing yourself to crypto/fiat price fluctuation risk. Example (let's assume 20% tax):
Early in the year you made profit on crypto-to-crypto trade of 1 btc, worth $20,000 at the time.
Later in the year you traded with loss of 0.5 btc, but bitcoin price fell down to $6,000, so you lost $3,000.
At the end of the year the rate is 1btc = $8,000.
You would think that since your total profit was 0.5 btc x $8,000 = $4,000 - your total tax would be $4,000 x 20% = $800. But taxman is only looking at fiat value at the time of the trades, so: 1st trade profit $20,000 less 2nd trade loss of $3,000 = $17,000 at 20% = $3,400.
So, in this case, tax is 85% of your actual profit.
Unless it falls under gambling?
In the UK day trading that falls under 'speculative' category is treated at par with gambling. You don't pay tax on gains, but you can't deduct losses. Source: https://www.daytrading.com/taxes/uk