How do I use a for loop to create a list of data frames, based off two existing data frames, in R?

I am VERY new to loops. Some of my loops have been successful, others... not so much.

I have some observed data (df_obs) that I'd like to test against my model predictions (df_pred).

MY CURRENT AIM: write a loop which makes a list of data frames, so that can I use this list in future loops assessing model performance. I will probably be back for help with THOSE loops...

YES: I do want a list of data frames. I'm working with 50+ species and have a bunch of tests to run on these values.

MAYBE: I think I want a for() loop, but if a different method is easier e.g. lapply(), I'm open to suggestions.

I've done my best to create a reproducible data set and code that mimics what I am working with:

#observed presence (1) and absence (0)
set.seed(733)
df_obs <- data.frame(plot = 1:10,
                    sp1 = sample(0:1, 10, replace = TRUE),
                    sp2 = sample(0:1, 10, replace = TRUE),
                    sp3 = sample(0:1, 10, replace = TRUE))

#predicted probability of occurrence (ranges from 0 to 1)
set.seed(733)
df_preds <- data.frame(plot = 1:10,
                       sp1 = runif(10, 0, 1),
                       sp2 = runif(10, 0, 1),
                       sp3 = runif(10, 0, 1))

sppcodes <- c("sp1", "sp2", "sp3")

test.eval.list <- vector("list", length = length(sppcodes))
names(test.eval.list) <- sppcodes


for(i in seq_along(sppcodes)){
  sppn <- sppcodes[i]

  plot = df_obs$plot
  obs = df_obs[,sppn]
  pred = df_preds[,sppn]

  df <- data.frame(plot, obs, pred) #produces dataframe as expected

  test.eval.list[sppn] <- df #problem seems to be here, it ends up assigning a vector of numbers...  
}

Could someone please help me understand why I am not ending up with a list of data frames, and give a correct way of doing so?

Please note - I know there are areas which could be done in a single line of code, I prefer this way of spreading the code out to understand which parts are/are not working.