Because the reaction ("like") isn't just for you (the note author). The reaction is also for the creator, to be able to go back and see the things they've liked in the past. The reactions may also be aggregated to provide signal to other observers.
Currently, "+" and "๐ค" are far and away the most popular reactions, but they're not the only ones. Here are some others that my node has seen in the last couple of days (with counts, to show relative frequency):
["+",259104]
["๐ค",141352]
["โ ๏ธ",1720]
["๐",911]
["๐",504]
["โค๏ธ",433]
["๐",401]
["๐ฅ",324]
["๐",248]
["๐ฎ",196]
["๐ญ",180]
["๐ค",125]
["๐ฅ",117]
["๐ฅ",109]
["โก๏ธ",105]
["๐คฃ",104]
["๐ฉ",96]
["๐ฝ",72]
["๐พ",70]
["๐ฎ",69]
["๐ง",68]
["๐ฎ",58]
["๐๐ป",54]
["๐",52]
["๐",51]
["๐ฆด",48]
["๐",48]
["๐",45]
["๐",43]
["๐ฏ",37]