Because it's a generic bracket included as a courtesy by AMD, as at the time of release there weren't any TR4 brackets in stock by liquid cooling manufacturers. Corsair makes a bracket for the H1xx series, it's $10.
https://www.corsair.com/us/en/Categories/Products/Accessories-|-Parts/PC-Components/Cooling/Hydro-Series-H100i-PRO-H115i-PRO-H150i-PRO-TR4-Mounting-Bracket-Kit/p/CW-8960054
