The Brainliest Answer!
They are made to have a broad and heavy base so as to make them more stable.

Hope this helps:)
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sry but myself aalima khan and u mark on diazrees..........
i think i give u the best reason thn the brainlist is mine but gave that to others.........
Sorry!!! But I think Diyazree's is more appropriate than yours. 
Diyazree's answer is short and sweet and nice!!!
okkkkkkkkk but thats ur tinking............
This is to reduce the chances of the burner accidentally tipping over. Knocking over a lit Bunsen burner could obviously have very bad results.