How did Buddha attain enlightenment without doing sadhana, seva, or satsang?
Who said that Buddha did not meditate? Meditation is sadhana!
Buddha also travelled to so many places and served the people. He listened to everyone's problems, and taught them meditation. Isn't this seva?
Who said he didn't do satsang? Every day they would sit and do satsang. All the Buddhist scriptures are from these satsang only. He sang and spoke in Pali (a Middle Indo-Aryan language).
He is the one who said, 'Bhavatu sabba mangalam, rakkhantu sabba devata' (May all beings be happy, and may all the Gods protect you).
In those days the language used was Pali, so whatever Buddha spoke and sang in Pali, is what has become the Buddhist scriptures today.
If you look at it that way, then you can say that Lord Krishna did do much. He recited the Bhagavad Gita.
In fact Lord Buddha did a lot. He meditated a lot, he fasted a lot. Whatever anyone told him to do, he did. After doing so much, when finally he got very tired, that is when he got enlightenment.
Buddha did a lot of sadhana. Krishna did not do much sadhana at all. He was mischievous; he did something here, and then he went and did something there. That is why it is called Krishna Leela (The Play of Krishna), and Buddha Katha (The Story of Buddha).