Interpret regression results

I was playing around with R built-in library mtcars and don't quite understand why I get following results.

attach(mtcars)
cor(mtcars[, 1:7])
mpg        cyl       disp         hp        drat         wt        qsec
mpg   1.0000000 -0.8521620 -0.8475514 -0.7761684  0.68117191 -0.8676594  0.41868403
cyl  -0.8521620  1.0000000  0.9020329  0.8324475 -0.69993811  0.7824958 -0.59124207
disp -0.8475514  0.9020329  1.0000000  0.7909486 -0.71021393  0.8879799 -0.43369788
hp   -0.7761684  0.8324475  0.7909486  1.0000000 -0.44875912  0.6587479 -0.70822339
drat  0.6811719 -0.6999381 -0.7102139 -0.4487591  1.00000000 -0.7124406  0.09120476
wt   -0.8676594  0.7824958  0.8879799  0.6587479 -0.71244065  1.0000000 -0.17471588
qsec  0.4186840 -0.5912421 -0.4336979 -0.7082234  0.09120476 -0.1747159  1.00000000


So I see that mpg (dependent variable) is correlated with six variables that I choose. Plot looks like this

If I regress mpg on, for example, hp then I have signigificant coefficient. However, if I include all variables in equation than I end up with insignificant coefficients for almost all betas:

> summary(lm(mpg ~ hp))

Call:
lm(formula = mpg ~ hp)

Residuals:
Min      1Q  Median      3Q     Max
-5.7121 -2.1122 -0.8854  1.5819  8.2360

Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 30.09886    1.63392  18.421  < 2e-16 ***
hp          -0.06823    0.01012  -6.742 1.79e-07 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 3.863 on 30 degrees of freedom
Multiple R-squared:  0.6024,    Adjusted R-squared:  0.5892
F-statistic: 45.46 on 1 and 30 DF,  p-value: 1.788e-07

> summary(lm(mpg ~ cyl + disp+ hp + drat + wt + qsec))

Call:
lm(formula = mpg ~ cyl + disp + hp + drat + wt + qsec)

Residuals:
Min      1Q  Median      3Q     Max
-3.9682 -1.5795 -0.4353  1.1662  5.5272

Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 26.30736   14.62994   1.798  0.08424 .
cyl         -0.81856    0.81156  -1.009  0.32282
disp         0.01320    0.01204   1.097  0.28307
hp          -0.01793    0.01551  -1.156  0.25846
drat         1.32041    1.47948   0.892  0.38065
wt          -4.19083    1.25791  -3.332  0.00269 **
qsec         0.40146    0.51658   0.777  0.44436
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 2.557 on 25 degrees of freedom
Multiple R-squared:  0.8548,    Adjusted R-squared:   0.82
F-statistic: 24.53 on 6 and 25 DF,  p-value: 2.45e-09


Why this is the case? Intuitively, mpg can be described by chosen variables. Also knowing about omitted variable bias, it is better to include all potential variables into equation. What conclusion should I make from this results?

• "it is better to include all potential variables into equation." No. Consider collinearity. Calculate the variance inflation factors. – Roland Oct 27 '16 at 11:14

Another important thing is that mpg is a discrete variable; so think if does it have sense to analyze the correlation between a continuous variable and a discrete one.