Because your explanation is incorrect.
If the premise of the OP is that the saints of revelation 20:4 are presently reigning  because the verb tenses of Ephesians 2:6 are past tense, but revelation 20:4 and ephesians 2:6 are different events, then using Ephesians 2:6 to prove revelation 20:4 SPECIFICALLY AGAINST PREMIL, is moot. 
1.) Let's say I'm a premil and I believe the reigning of saints in revelation 20:4 is a literal future reality when the saints are raised again at the 2nd coming and sit on thrones to rule over the nations. 
2.) However, you being and Amil, say no, it's a literal event about the souls of believers being in heaven to sit on thrones and rule with Christ following the 1st coming. 
3.) You then proceed to provide Ephesians 2:6 to prove revelation 20:4 is a present reality of souls reigning in heaven with Christ, because you say "look, the verb tenses are past and show that we are already seated in heaven, thus proving revelation 20:4 should be understood as Amil". 
4.) I, as the premil, will say, "great observation on the verb tenses, so ephesians 2:6 is about souls being in heaven to reign?"
5.) You, the amil, say "NO, it's NOT about souls being in heaven". 
6.) I, as the premil, now being very confused say "what? then how does ephesians 2:6, which is is NOT about souls being in  heaven with Christ, prove revelation 20:4 is about souls being in heaven reigning with Christ?"