This is a two-fold answer - your "winnings" are not taxed with licensed German bookmakers, but your initial stake might be taxed by 5.3%.
Since 2012, all sports and casino bets are taxed at 5.3% of the amount wagered. That sum is required by law from the bookmakers themselves.
Some online bookmakers fully or partially cover the tax instead of the players, while others transfer that cost on them - it is a personal business choice.
With those betting sites that do pay that tax themselves, you get higher payout margins and you get to keep more of your profits.
Some of the betting sites in Germany, which offer to pay the tax for their players are Tipwin, Bet3000 and Winamax.