I understand that by "civil law systems" the OP means to refer to civil (vs criminal) cases, not to the civil law (vs common law) systems.
The standard for proof of "guilt" (which is not actually called "guilt") is indeed "much lower" in civil cases, but the reason for that is not that it has to be so, but that the standard of proof in criminal cases has to be much higher.
In civil cases none of the parties face criminal conviction. One party will just lose some money or property, will have to perform some work, will lose some opportunities etc. So, initially, both parties play the same game and each of them is in to lose it down to a similar level of peril.
What follows is that, at the baseline, neither of the two parties should be in more advantageous position to prove their rightness than the other. In other words, if any of them proves that they are more than 50% likely to be right than the other, they should win.
Indeed, why would one party have to prove their rightness beyond reasonable doubt? If it was so, it would mean that the other party would effectively have to prove their rightness to only a small degree, which would be utterly unjust.
lot of guarentees like right avainst self incrimination are also not available in civil law systems. what is the reason behind this ?
False. If saying something from a witness stand would risk you being prosecuted, you can refuse to say it regardless of whether you are giving evidence in a civil or a criminal case.