How can you provide complete secrecy of the voter's choice? Let's say I want to buy a vote. In the current system, the person I am paying disappears into a booth, and I actually have no idea how they voted.
BTW, physical presence system is easily gamed too: they intimidate you to vote for Edinaya Rossiya (ruling party in Russia) or loose the job (teachers, budget workers, government-owned companies, etc -- lots of people) and demand you to make a photo of your ballot with correct check mark. Guess what? 99% of people complied with that... the remaining 1% found some tricks like placing a short thread on empty square and the photo then mark another party. But these were minority.
Even physical voting security is hard, because they game it in many, many ways and it must be done via very strict procedure with free media watching that. Online voting is pure madness.