Maybe, because people won't be expecting, right then and there:

Matthew 24:42 Watch therefore: for ye know not what hour your Lord doth come.

  44 Therefore be ye also ready: for in such an hour as ye think not the Son of man cometh.

  50 The lord of that servant shall come in a day when he looketh not for him, and in an hour that he is not aware of,

Maybe that's what the comparisons were about, people will be going about their daily lives, like they were in the past:

Luke 17:26 And as it was in the days of Noe, so shall it be also in the days of the Son of man.

  27 They did eat, they drank, they married wives, they were given in marriage, until the day that Noe entered into the ark, and the flood came, and destroyed them all.

  28 Likewise also as it was in the days of Lot; they did eat, they drank, they bought, they sold, they planted, they builded;

Everyone then was expecting the day to be like any other day.

Perhaps that is what it will also be like, they will be going about their daily lives:

Luke 17:34 I tell you, in that night there shall be two men in one bed; the one shall be taken, and the other shall be left.

  35 Two women shall be grinding together; the one shall be taken, and the other left.

  36 Two men shall be in the field; the one shall be taken, and the other left.

Thus it will be when least expected.